Table of Contents
INFORMATION_SCHEMA
Questions
28.1.1: When did MySQL 5.0 become production-ready (GA)?
28.1.2: Can MySQL 5.0 do subqueries?
28.1.3: Can MySQL 5.0 peform multiple-table inserts, updates, and deletes?
28.1.4: Does MySQL 5.0 have a Query Cache? Does it work on Server, Instance or Database?
28.1.5: Does MySQL 5.0 have Sequences?
28.1.6:
Does MySQL 5.0 have a
NOW()
function with
fractions of seconds?
28.1.7: Does MySQL 5.0 work with multi-core processors?
28.1.8: Is there a hot backup tool for MyISAM like InnoDB Hot Backup?
28.1.9: Have there been there any improvements in error reporting when foreign keys fail? Does MySQL now report which column and reference failed?
28.1.10: Can MySQL 5.0 perform ACID transactions?
Questions and Answers
28.1.1: When did MySQL 5.0 become production-ready (GA)?
MySQL 5.0.15 was released for production use on 19 October 2005. We are now working on MySQL 5.1, which is currently in beta.
28.1.2: Can MySQL 5.0 do subqueries?
Yes. See Section 12.2.8, “Subquery Syntax”.
28.1.3: Can MySQL 5.0 peform multiple-table inserts, updates, and deletes?
Yes. For the syntax required to perform multiple-table
updates, see Section 12.2.10, “UPDATE
Syntax”; for that required to
perform multiple-table deletes, see
Section 12.2.1, “DELETE
Syntax”.
A multiple-table insert can be accomplished using a trigger
whose FOR EACH ROW
clause contains
multiple INSERT
statements within a
BEGIN ... END
block. See
Section 19.3, “Using Triggers”.
28.1.4: Does MySQL 5.0 have a Query Cache? Does it work on Server, Instance or Database?
Yes. The query cache operates on the server level, caching complete result sets matched with the original query string. If an exactly identical query is made (which often happens, particularly in web applications), no parsing or execution is necessary; the result is sent directly from the cache. Various tuning options are available. See Section 7.5.4, “The MySQL Query Cache”.
28.1.5: Does MySQL 5.0 have Sequences?
No. However, MySQL has an AUTO_INCREMENT
system, which in MySQL 5.0 can also handle
inserts in a multi-master replication setup. With the
--auto-increment-increment
and
--auto-increment-offset
startup options,
you can set each server to generate auto-increment values
that don't conflict with other servers. The
--auto-increment-increment
value should be
greater than the number of servers, and each server should
have a unique offset.
28.1.6:
Does MySQL 5.0 have a
NOW()
function with
fractions of seconds?
No. This is on the MySQL roadmap as a “rolling feature”. This means that it is not a flagship feature, but will be implemented, development time permitting. Specific customer demand may change this scheduling.
However, MySQL does parse time strings with a fractional
component. See Section 10.3.2, “The TIME
Type”.
28.1.7: Does MySQL 5.0 work with multi-core processors?
Yes. MySQL is fully multi-threaded, and will make use of multiple CPUs, provided that the operating system supports them.
28.1.8: Is there a hot backup tool for MyISAM like InnoDB Hot Backup?
This is currently under development for a future MySQL release.
28.1.9: Have there been there any improvements in error reporting when foreign keys fail? Does MySQL now report which column and reference failed?
The foreign key support in InnoDB
has
seen improvements in each major version of MySQL. Foreign
key support generic to all storage engines is scheduled for
MySQL 6.x; this should resolve any inadequacies in the
current storage engine specific implementation.
28.1.10: Can MySQL 5.0 perform ACID transactions?
Yes. All current MySQL versions support transactions. The
InnoDB
storage engine offers full ACID
transactions with row-level locking, multi-versioning,
non-locking repeatable reads, and all four SQL standard
isolation levels.
The NDB
storage engine supports the
READ COMMITTED
transaction isolation
level only.
Questions
28.2.1: Where can I obtain complete documentation for MySQL storage engines?
28.2.2: Are there any new storage engines in MySQL 5.0?
28.2.3: Have any storage engines been removed in MySQL 5.0?
28.2.4:
What are the unique benefits of the
ARCHIVE
storage engine?
28.2.5: Do the new features in MySQL 5.0 apply to all storage engines?
Questions and Answers
28.2.1: Where can I obtain complete documentation for MySQL storage engines?
See Chapter 13, Storage Engines. That chapter contains
information about all MySQL storage engines except for the
NDB
storage engine used for MySQL
Cluster; NDB
is covered in
Chapter 16, MySQL Cluster.
28.2.2: Are there any new storage engines in MySQL 5.0?
Yes. The FEDERATED
storage engine, new in
MySQL 5.0, allows the server to access tables
on other (remote) servers. See
Section 13.7, “The FEDERATED
Storage Engine”.
28.2.3: Have any storage engines been removed in MySQL 5.0?
Yes. MySQL 5.0 no longer supports the
ISAM
storage engine. If you have any
existing ISAM
tables from previous
versions of MySQL, you should convert these to
MyISAM
before upgrading to MySQL
5.0.
28.2.4:
What are the unique benefits of the
ARCHIVE
storage engine?
The ARCHIVE
storage engine is ideally
suited for storing large amounts of data without indexes; it
has a very small footprint, and performs selects using table
scans. See Section 13.8, “The ARCHIVE
Storage Engine”, for
details.
28.2.5: Do the new features in MySQL 5.0 apply to all storage engines?
The general new features such as views, stored procedures,
triggers, INFORMATION_SCHEMA
, precision
math (DECIMAL
column type), and the
BIT
column type, apply to all storage
engines. There are also additions and changes for specific
storage engines.
Questions
28.3.1: What are server SQL modes?
28.3.2: How many server SQL modes are there?
28.3.3: How do you determine the server SQL mode?
28.3.4: Is the mode dependent on the database or connection?
28.3.5: Can the rules for strict mode be extended?
28.3.6: Does strict mode impact performance?
28.3.7: What is the default server SQL mode when My SQL 5.0 is installed?
Questions and Answers
28.3.1: What are server SQL modes?
Server SQL modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers. The MySQL Server apply these modes individually to different clients. For more information, see Section 5.1.6, “SQL Modes”.
28.3.2: How many server SQL modes are there?
Each mode can be independently switched on and off. See Section 5.1.6, “SQL Modes”, for a complete list of available modes.
28.3.3: How do you determine the server SQL mode?
You can set the default SQL mode (for
mysqld startup) with the
--sql-mode
option. Using the statement
SET [SESSION|GLOBAL]
sql_mode='
, you
can change the settings from within a connection, either
locally to the connection, or to take effect globally. You
can retrieve the current mode by issuing a modes
'SELECT
@@sql_mode
statement.
28.3.4: Is the mode dependent on the database or connection?
A mode is not linked to a particular database. Modes can be
set locally to the session (connection), or globally for the
server. you can change these settings using SET
[SESSION|GLOBAL]
sql_mode='
.
modes
'
28.3.5: Can the rules for strict mode be extended?
When we refer to strict mode, we mean a
mode where at least one of the modes
TRADITIONAL
,
STRICT_TRANS_TABLES
, or
STRICT_ALL_TABLES
is enabled. Options can
be combined, so you can add additional restrictions to a
mode. See Section 5.1.6, “SQL Modes”, for more
information.
28.3.6: Does strict mode impact performance?
The intensive validation of input data that some settings requires more time than if the validation is not done. While the performance impact is not that great, if you do not require such validation (perhaps your application already handles all of this), then MySQL gives you the option of leaving strict mode disabled. However — if you do require it — strict mode can provide such validation.
28.3.7: What is the default server SQL mode when My SQL 5.0 is installed?
By default, no special modes are enabled. See Section 5.1.6, “SQL Modes”, for information about all available modes and MySQL's default behavior.
Questions
28.4.1: Does MySQL 5.0 support stored procedures?
28.4.2: Where can I find documentation for MySQL stored procedures and stored functions?
28.4.3: Is there a discussion forum for MySQL stored procedures?
28.4.4: Where can I find the ANSI SQL 2003 specification for stored procedures?
28.4.5: How do you manage stored routines?
28.4.6: Is there a way to view all stored procedures and stored functions in a given database?
28.4.7: Where are stored procedures stored?
28.4.8: Is it possible to group stored procedures or stored functions into packages?
28.4.9: Can a stored procedure call another stored procedure?
28.4.10: Can a stored procedure call a trigger?
28.4.11: Can a stored procedure access tables?
28.4.12: Do stored procedures have a statement for raising application errors?
28.4.13: Do stored procedures provide exception handling?
28.4.14: Can MySQL 5.0 stored routines return result sets?
28.4.15:
Is WITH RECOMPILE
supported for stored
procedures?
28.4.16:
Is there a MySQL equivalent to using
mod_plsql
as a gateway on Apache to talk
directly to a stored procedure in the database?
28.4.17: Can I pass an array as input to a stored procedure?
28.4.18:
Can I pass a cursor as an IN
parameter to
a stored procedure?
28.4.19:
Can I return a cursor as an OUT
parameter
from a stored procedure?
28.4.20: Can I print out a variable's value within a stored routine for debugging purposes?
28.4.21: Can I commit or roll back transactions inside a stored procedure?
28.4.22: Do MySQL 5.0 stored procedures and functions work with replication?
28.4.23: Are stored procedures and functions created on a master server replicated to a slave?
28.4.24: How are actions that take place inside stored procedures and functions replicated?
28.4.25: Are there special security requirements for using stored procedures and functions together with replication?
28.4.26: What limitations exist for replicating stored procedure and function actions?
28.4.27: Do the preceding limitations affect MySQL's ability to do point-in-time recovery?
28.4.28: What is being done to correct the aforementioned limitations?
Questions and Answers
28.4.1: Does MySQL 5.0 support stored procedures?
Yes. MySQL 5.0 supports two types of stored routines — stored procedures and stored functions.
28.4.2: Where can I find documentation for MySQL stored procedures and stored functions?
See Chapter 18, Stored Procedures and Functions.
28.4.3: Is there a discussion forum for MySQL stored procedures?
Yes. See http://forums.mysql.com/list.php?98.
28.4.4: Where can I find the ANSI SQL 2003 specification for stored procedures?
Unfortunately, the official specifications are not freely available (ANSI makes them available for purchase). However, there are books — such as SQL-99 Complete, Really by Peter Gulutzan and Trudy Pelzer — which give a comprehensive overview of the standard, including coverage of stored procedures.
28.4.5: How do you manage stored routines?
It is always good practice to use a clear naming scheme for
your stored routines. You can manage stored procedures with
CREATE [FUNCTION|PROCEDURE]
,
ALTER [FUNCTION|PROCEDURE]
, DROP
[FUNCTION|PROCEDURE]
, and SHOW CREATE
[FUNCTION|PROCEDURE]
. You can obtain information
about existing stored procedures using the
ROUTINES
table in the
INFORMATION_SCHEMA
database (see
Section 21.14, “The INFORMATION_SCHEMA ROUTINES
Table”).
28.4.6: Is there a way to view all stored procedures and stored functions in a given database?
Yes. For a database named dbname
,
use this query on the
INFORMATION_SCHEMA.ROUTINES
table:
SELECT ROUTINE_TYPE, ROUTINE_NAME
FROM INFORMATION_SCHEMA.ROUTINES
WHERE ROUTINE_SCHEMA='dbname
';
For more information, see Section 21.14, “The INFORMATION_SCHEMA ROUTINES
Table”.
The body of a stored routine can be viewed using
SHOW CREATE FUNCTION
(for a stored
function) or SHOW CREATE PROCEDURE
(for a
stored procedure). See
Section 12.5.4.5, “SHOW CREATE PROCEDURE
and SHOW CREATE
FUNCTION
Syntax”, for more
information.
28.4.7: Where are stored procedures stored?
In the proc
table of the
mysql
system database. However, you
should not access the tables in the system database
directly. Instead, use SHOW CREATE
FUNCTION
to obtain information about stored
functions, and SHOW CREATE PROCEDURE
to
obtain information about stored procedures. See
Section 12.5.4.5, “SHOW CREATE PROCEDURE
and SHOW CREATE
FUNCTION
Syntax”, for more
information about these statements.
You can also query the ROUTINES
table in
the INFORMATION_SCHEMA
database —
see Section 21.14, “The INFORMATION_SCHEMA ROUTINES
Table”, for information about
this table.
28.4.8: Is it possible to group stored procedures or stored functions into packages?
No. This is not supported in MySQL 5.0.
28.4.9: Can a stored procedure call another stored procedure?
Yes.
28.4.10: Can a stored procedure call a trigger?
A stored procedure can execute an SQL statement, such as an
UPDATE
, that causes a trigger to fire.
28.4.11: Can a stored procedure access tables?
Yes. A stored procedure can access one or more tables as required.
28.4.12: Do stored procedures have a statement for raising application errors?
Not in MySQL 5.0. We intend to implement the
SQL standard SIGNAL
and
RESIGNAL
statements in a future MySQL
release.
28.4.13: Do stored procedures provide exception handling?
MySQL implements HANDLER
definitions
according to the SQL standard. See
Section 18.2.8.2, “DECLARE
Handlers”, for details.
28.4.14: Can MySQL 5.0 stored routines return result sets?
Stored procedures can, but stored
functions cannot. If you perform an ordinary
SELECT
inside a stored procedure, the
result set is returned directly to the client. You need to
use the MySQL 4.1 (or above) client-server protocol for this
to work. This means that — for instance — in
PHP, you need to use the mysqli
extension
rather than the old mysql
extension.
28.4.15:
Is WITH RECOMPILE
supported for stored
procedures?
Not in MySQL 5.0.
28.4.16:
Is there a MySQL equivalent to using
mod_plsql
as a gateway on Apache to talk
directly to a stored procedure in the database?
There is no equivalent in MySQL 5.0.
28.4.17: Can I pass an array as input to a stored procedure?
Not in MySQL 5.0.
28.4.18:
Can I pass a cursor as an IN
parameter to
a stored procedure?
In MySQL 5.0, cursors are available inside stored procedures only.
28.4.19:
Can I return a cursor as an OUT
parameter
from a stored procedure?
In MySQL 5.0, cursors are available inside
stored procedures only. However, if you do not open a cursor
on a SELECT
, the result will be sent
directly to the client. You can also SELECT
INTO
variables. See Section 12.2.7, “SELECT
Syntax”.
28.4.20: Can I print out a variable's value within a stored routine for debugging purposes?
Yes, you can do this in a stored
procedure, but not in a stored function. If you
perform an ordinary SELECT
inside a
stored procedure, the result set is returned directly to the
client. You will need to use the MySQL 4.1 (or above)
client-server protocol for this to work. This means that
— for instance — in PHP, you need to use the
mysqli
extension rather than the old
mysql
extension.
28.4.21: Can I commit or roll back transactions inside a stored procedure?
Yes. However, you cannot perform transactional operations within a stored function.
28.4.22: Do MySQL 5.0 stored procedures and functions work with replication?
Yes, standard actions carried out in stored procedures and functions are replicated from a master MySQL server to a slave server. There are a few limitations that are described in detail in Section 18.5, “Binary Logging of Stored Routines and Triggers”.
28.4.23: Are stored procedures and functions created on a master server replicated to a slave?
Yes, creation of stored procedures and functions carried out
through normal DDL statements on a master server are
replicated to a slave, so the objects will exist on both
servers. ALTER
and
DROP
statements for stored procedures and
functions are also replicated.
28.4.24: How are actions that take place inside stored procedures and functions replicated?
MySQL records each DML event that occurs in a stored procedure and replicates those individual actions to a slave server. The actual calls made to execute stored procedures are not replicated.
Stored functions that change data are logged as function invocations, not as the DML events that occur inside each function.
28.4.25: Are there special security requirements for using stored procedures and functions together with replication?
Yes. Because a slave server has authority to execute any statement read from a master's binary log, special security constraints exist for using stored functions with replication. If replication or binary logging in general (for the purpose of point-in-time recovery) is active, then MySQL DBAs have two security options open to them:
Any user wishing to create stored functions must be
granted the SUPER
privilege.
Alternatively, a DBA can set the
log_bin_trust_function_creators
system variable to 1, which enables anyone with the
standard CREATE ROUTINE
privilege
to create stored functions.
28.4.26: What limitations exist for replicating stored procedure and function actions?
Non-deterministic (random) or time-based actions embedded in
stored procedures may not replicate properly. By their very
nature, randomly produced results are not predictable and
cannot be exactly reproduced, and therefore, random actions
replicated to a slave will not mirror those performed on a
master. Note that declaring stored functions to be
DETERMINISTIC
or setting the
log_bin_trust_function_creators
system
variable to 0 will not allow random-valued operations to be
invoked.
In addition, time-based actions cannot be reproduced on a slave because the timing of such actions in a stored procedure is not reproducible through the binary log used for replication. It records only DML events and does not factor in timing constraints.
Finally, non-transactional tables for which errors occur
during large DML actions (such as bulk inserts) may
experience replication issues in that a master may be
partially updated from DML activity, but no updates are done
to the slave because of the errors that occurred. A
workaround is for a function's DML actions to be carried out
with the IGNORE
keyword so that updates
on the master that cause errors are ignored and updates that
do not cause errors are replicated to the slave.
28.4.27: Do the preceding limitations affect MySQL's ability to do point-in-time recovery?
The same limitations that affect replication do affect point-in-time recovery.
28.4.28: What is being done to correct the aforementioned limitations?
MySQL 5.1 implements row-based replication, which resolves the limitations mentioned earlier.
We do not plan to backport row-based replication to MySQL 5.0. For additional information, see Replication Formats, in the MySQL 5.1 Manual.
Questions
28.5.1: Where can I find the documentation for MySQL 5.0 triggers?
28.5.2: Is there a discussion forum for MySQL Triggers?
28.5.3: Does MySQL 5.0 have statement-level or row-level triggers?
28.5.4: Are there any default triggers?
28.5.5: How are triggers managed in MySQL?
28.5.6: Is there a way to view all triggers in a given database?
28.5.7: Where are triggers stored?
28.5.8: Can a trigger call a stored procedure?
28.5.9: Can triggers access tables?
28.5.10: Can triggers call an external application through a UDF?
28.5.11: Is possible for a trigger to update tables on a remote server?
28.5.12: Do triggers work with replication?
28.5.13: How are actions carried out through triggers on a master replicated to a slave?
Questions and Answers
28.5.1: Where can I find the documentation for MySQL 5.0 triggers?
See Chapter 19, Triggers.
28.5.2: Is there a discussion forum for MySQL Triggers?
Yes. It is available at http://forums.mysql.com/list.php?99.
28.5.3: Does MySQL 5.0 have statement-level or row-level triggers?
In MySQL 5.0, all triggers are FOR
EACH ROW
— that is, the trigger is activated
for each row that is inserted, updated, or deleted. MySQL
5.0 does not support triggers using
FOR EACH STATEMENT
.
28.5.4: Are there any default triggers?
Not explicitly. MySQL does have specific special behavior
for some TIMESTAMP
columns, as well as
for columns which are defined using
AUTO_INCREMENT
.
28.5.5: How are triggers managed in MySQL?
In MySQL 5.0, triggers can be created using the
CREATE TRIGGER
statement, and dropped
using DROP TRIGGER
. See
Section 19.1, “CREATE TRIGGER
Syntax”, and
Section 19.2, “DROP TRIGGER
Syntax”, for more about these
statements.
Information about triggers can be obtained by querying the
INFORMATION_SCHEMA.TRIGGERS
table. See
Section 21.16, “The INFORMATION_SCHEMA TRIGGERS
Table”.
28.5.6: Is there a way to view all triggers in a given database?
Yes. You can obtain a listing of all triggers defined on
database dbname
using a query on the
INFORMATION_SCHEMA.TRIGGERS
table such as
the one shown here:
SELECT TRIGGER_NAME, EVENT_MANIPULATION, EVENT_OBJECT_TABLE, ACTION_STATEMENT
FROM INFORMATION_SCHEMA.TRIGGERS
WHERE TRIGGER_SCHEMA='dbname
';
For more information about this table, see
Section 21.16, “The INFORMATION_SCHEMA TRIGGERS
Table”.
You can also use the SHOW TRIGGERS
statement, which is specific to MySQL. See
Section 12.5.4.26, “SHOW TRIGGERS
Syntax”.
28.5.7: Where are triggers stored?
Triggers for a table are currently stored in
.TRG
files, with one such file one per
table.
28.5.8: Can a trigger call a stored procedure?
Yes.
28.5.9: Can triggers access tables?
A trigger can access both old and new data in its own table. Through a stored procedure, or a multiple-table update or delete statement, a trigger can also affect other tables.
28.5.10: Can triggers call an external application through a UDF?
No, not at present.
28.5.11: Is possible for a trigger to update tables on a remote server?
Yes. A table on a remote server could be updated using the
FEDERATED
storage engine. (See
Section 13.7, “The FEDERATED
Storage Engine”).
28.5.12: Do triggers work with replication?
Triggers and replication in MySQL 5.0 work in the same wasy as in most other database engines: Actions carried out through triggers on a master are not replicated to a slave server. Instead, triggers that exist on tables that reside on a MySQL master server need to be created on the corresponding tables on any MySQL slave servers so that the triggers activate on the slaves as well as the master.
28.5.13: How are actions carried out through triggers on a master replicated to a slave?
First, the triggers that exist on a master must be
re-created on the slave server. Once this is done, the
replication flow works as any other standard DML statement
that participates in replication. For example, consider a
table EMP
that has an
AFTER
insert trigger, which exists on a
master MySQL server. The same EMP
table
and AFTER
insert trigger exist on the
slave server as well. The replication flow would be:
An INSERT
statement is made to
EMP
.
The AFTER
trigger on
EMP
activates.
The INSERT
statement is written to
the binary log.
The replication slave picks up the
INSERT
statement to
EMP
and executes it.
The AFTER
trigger on
EMP
that exists on the slave
activates.
Questions
28.6.1: Where can I find documentation covering MySQL Views?
28.6.2: Is there a discussion forum for MySQL Views?
28.6.3: What happens to a view if an underlying table is dropped or renamed?
28.6.4: Does MySQL 5.0 have table snapshots?
28.6.5: Does MySQL 5.0 have materialized views?
28.6.6: Can you insert into views that are based on joins?
Questions and Answers
28.6.1: Where can I find documentation covering MySQL Views?
See Chapter 20, Views.
28.6.2: Is there a discussion forum for MySQL Views?
Yes. See http://forums.mysql.com/list.php?100
28.6.3: What happens to a view if an underlying table is dropped or renamed?
After a view has been created, it is possible to drop or
alter a table or view to which the definition refers. To
check a view definition for problems of this kind, use the
CHECK TABLE
statement. (See
Section 12.5.2.3, “CHECK TABLE
Syntax”.)
28.6.4: Does MySQL 5.0 have table snapshots?
No.
28.6.5: Does MySQL 5.0 have materialized views?
No.
28.6.6: Can you insert into views that are based on joins?
It is possible, provided that your INSERT
statement has a column list that makes it clear there's only
one table involved.
You cannot insert into multiple tables with a single insert on a view.
Questions
28.7.1:
Where can I find documentation for the MySQL
INFORMATION_SCHEMA
database?
28.7.2:
Is there a discussion forum for
INFORMATION_SCHEMA
?
28.7.3:
Where can I find the ANSI SQL 2003 specification for
INFORMATION_SCHEMA
?
28.7.4:
What is the difference between the Oracle Data Dictionary
and MySQL's INFORMATION_SCHEMA
?
28.7.5:
Can I add to or otherwise modify the tables found in the
INFORMATION_SCHEMA
database?
Questions and Answers
28.7.1:
Where can I find documentation for the MySQL
INFORMATION_SCHEMA
database?
See Chapter 21, INFORMATION_SCHEMA
Tables
28.7.2:
Is there a discussion forum for
INFORMATION_SCHEMA
?
See http://forums.mysql.com/list.php?101.
28.7.3:
Where can I find the ANSI SQL 2003 specification for
INFORMATION_SCHEMA
?
Unfortunately, the official specifications are not freely
available. (ANSI makes them available for purchase.)
However, there are books available — such as
SQL-99 Complete, Really by Peter
Gulutzan and Trudy Pelzer — which give a comprehensive
overview of the standard, including
INFORMATION_SCHEMA
.
28.7.4:
What is the difference between the Oracle Data Dictionary
and MySQL's INFORMATION_SCHEMA
?
Both Oracle and MySQL provide metadata in tables. However,
Oracle and MySQL use different table names and column names.
MySQL's implementation is more similar to those found in DB2
and SQL Server, which also support
INFORMATION_SCHEMA
as defined in the SQL
standard.
28.7.5:
Can I add to or otherwise modify the tables found in the
INFORMATION_SCHEMA
database?
No. Since applications may rely on a certain standard
structure, this should not be modified. For this reason,
MySQL AB cannot support bugs or other issues which
result from modifying INFORMATION_SCHEMA
tables or data.
Questions
Questions and Answers
28.8.1: Where can I find information on how to migrate from MySQL 4.1 to MySQL 5.0?
For detailed upgrade information, see Section 2.4.17, “Upgrading MySQL”. We recommend that you do not skip a major version when upgrading, but rather complete the process in steps, upgrading from one major version to the next in each step. This may seem more complicated, but it will you save time and trouble — if you encounter problems during the upgrade, their origin will be easier to identify, either by you or — if you have a MySQL Enterprise subscription — by MySQL support.
28.8.2: How has storage engine (table type) support changed in MySQL 5.0 from previous versions?
Storage engine support has changed as follows:
Support for ISAM
tables was removed
in MySQL 5.0 and you should now use the
MyISAM
storage engine in place of
ISAM
. To convert a table
tblname
from
ISAM
to MyISAM
,
simply issue a statement such as this one:
ALTER TABLE tblname
ENGINE=MYISAM;
Internal RAID
for
MyISAM
tables was also removed in
MySQL 5.0. This was formerly used to allow large
tables in file systems that did not support file sizes
greater than 2GB. All modern file systems allow for
larger tables; in addition, there are now other
solutions such as MERGE
tables and
views.
The VARCHAR
column type now retains
trailing spaces in all storage engines.
MEMORY
tables (formerly known as
HEAP
tables) can also contain
VARCHAR
columns.
Questions
28.9.1: Where can I find documentation that addresses security issues for MySQL?
28.9.2: Does MySQL 5.0 have native support for SSL?
28.9.3: Is SSL support be built into MySQL binaries, or must I recompile the binary myself to enable it?
28.9.4: Does MySQL 5.0 have built-in authentication against LDAP directories?
28.9.5: Does MySQL 5.0 include support for Roles Based Access Control (RBAC)?
Questions and Answers
28.9.1: Where can I find documentation that addresses security issues for MySQL?
The best place to start is Section 5.3, “General Security Issues”.
Other portions of the MySQL Documentation which you may find useful with regard to specific security concerns include the following:
MySQL Enterprise The MySQL Enterprise Monitor enforces best practices for maximizing the security of your servers. For more information see http://www.mysql.com/products/enterprise/advisors.html.
28.9.2: Does MySQL 5.0 have native support for SSL?
Most 5.0 binaries have support for SSL connections between the client and server. We can't currently build with the new YaSSL library everywhere, as it's still quite new and does not compile on all platforms yet. See Section 5.5.7, “Using Secure Connections”.
You can also tunnel a connection via SSH, if (for instance) if the client application doesn't support SSL connections. For an example, see Section 5.5.7.5, “Connecting to MySQL Remotely from Windows with SSH”.
28.9.3: Is SSL support be built into MySQL binaries, or must I recompile the binary myself to enable it?
Most 5.0 binaries have SSL enabled for client-server connections that are secured, authenticated, or both. However, the YaSSL library currently does not compile on all platforms. See Section 5.5.7, “Using Secure Connections”, for a complete listing of supported and unsupported platforms.
28.9.4: Does MySQL 5.0 have built-in authentication against LDAP directories?
No. Support for external authentication methods is on the MySQL roadmap as a “rolling feature”, which means that we plan to implement it in the future, but we have not yet determined when this will be done.
28.9.5: Does MySQL 5.0 include support for Roles Based Access Control (RBAC)?
No. Support for roles is on the MySQL roadmap as a “rolling feature”, which means that we plan to implement it in the future, but we have not yet determined when this will be done.
In the following section, we provide answers to questions that are
most frequently asked about MySQL Cluster and the
NDB
storage engine.
Questions
28.10.1: What does “NDB” mean?
28.10.2: What's the difference in using Cluster vs using replication?
28.10.3: Do I need to do any special networking to run Cluster? How do computers in a cluster communicate?
28.10.4: How many computers do I need to run a cluster, and why?
28.10.5: What do the different computers do in a MySQL Cluster?
28.10.6: With which operating systems can I use Cluster?
28.10.7: What are the hardware requirements for running MySQL Cluster?
28.10.8: How much RAM do I need? Is it possible to use disk memory at all?
28.10.9: What filesystems can I use with MySQL Cluster? What about network filesystems or network shares?
28.10.10: Can I run MySQL Cluster nodes inside virtual machines (such as those created by VMWare, Parallels, or Xen)?
28.10.11: I'm trying to populate a Cluster database. The loading process terminates prematurely and I get an error message like this one:
ERROR 1114: The table 'my_cluster_table' is full
Why is this happening?
28.10.12: MySQL Cluster uses TCP/IP. Does this mean that I can run it over the Internet, with one or more nodes in remote locations?
28.10.13: Do I have to learn a new programming or query language to use MySQL Cluster?
28.10.14: How do I find out what an error or warning message means when using MySQL Cluster?
28.10.15: Is MySQL Cluster transaction-safe? What isolation levels are supported?
28.10.16: What storage engines are supported by MySQL Cluster?
28.10.17: Which versions of the MySQL software support Cluster? Do I have to compile from source?
28.10.18: In the event of a catastrophic failure — say, for instance, the whole city loses power and my UPS fails — would I lose all my data?
28.10.19:
Is it possible to use FULLTEXT
indexes
with Cluster?
28.10.20: Can I run multiple nodes on a single computer?
28.10.21: Can I add nodes to a cluster without restarting it?
28.10.22: Are there any limitations that I should be aware of when using MySQL Cluster?
28.10.23: How do I import an existing MySQL database into a cluster?
28.10.24: How do cluster nodes communicate with one another?
28.10.25: What is an arbitrator?
28.10.26: What data types are supported by MySQL Cluster?
28.10.27: How do I start and stop MySQL Cluster?
28.10.28: What happens to cluster data when the cluster is shut down?
28.10.29: Is it helpful to have more than one management node for a cluster?
28.10.30: Can I mix different kinds of hardware and operating systems in one MySQL Cluster?
28.10.31: Can I run two data nodes on a single host? Two SQL nodes?
28.10.32: Can I use hostnames with MySQL Cluster?
28.10.33: How do I handle MySQL users in a Cluster having multiple MySQL servers?
Questions and Answers
28.10.1: What does “NDB” mean?
This stands for
“Network
Database”.
NDB
(also known as NDB
Cluster
or NDBCLUSTER
) is the
storage engine that enables clustering in MySQL.
28.10.2: What's the difference in using Cluster vs using replication?
In a replication setup, a master MySQL server updates one or
more slaves. Transactions are committed sequentially, and a
slow transaction can cause the slave to lag behind the
master. This means that if the master fails, it is possible
that the slave might not have recorded the last few
transactions. If a transaction-safe engine such as
InnoDB
is being used, a transaction will
either be complete on the slave or not applied at all, but
replication does not guarantee that all data on the master
and the slave will be consistent at all times. In MySQL
Cluster, all data nodes are kept in synchrony, and a
transaction committed by any one data node is committed for
all data nodes. In the event of a data node failure, all
remaining data nodes remain in a consistent state.
In short, whereas standard MySQL replication is asynchronous, MySQL Cluster is synchronous.
We have implemented (asynchronous) replication for Cluster in MySQL 5.1. This includes the capability to replicate both between two clusters, and from a MySQL cluster to a non-Cluster MySQL server. However, we do not plan to backport this functionality to MySQL 5.0.
28.10.3: Do I need to do any special networking to run Cluster? How do computers in a cluster communicate?
MySQL Cluster is intended to be used in a high-bandwidth environment, with computers connecting via TCP/IP. Its performance depends directly upon the connection speed between the cluster's computers. The minimum connectivity requirements for Cluster include a typical 100-megabit Ethernet network or the equivalent. We recommend you use gigabit Ethernet whenever available.
The faster SCI protocol is also supported, but requires special hardware. See Section 16.11, “Using High-Speed Interconnects with MySQL Cluster”, for more information about SCI.
28.10.4: How many computers do I need to run a cluster, and why?
A minimum of three computers is required to run a viable cluster. However, the minimum recommended number of computers in a MySQL Cluster is four: one each to run the management and SQL nodes, and two computers to serve as data nodes. The purpose of the two data nodes is to provide redundancy; the management node must run on a separate machine to guarantee continued arbitration services in the event that one of the data nodes fails.
To provide increased throughput and high availability, you should use multiple SQL nodes (MySQL Servers connected to the cluster). It is also possible (although not strictly necessary) to run multiple management servers.
28.10.5: What do the different computers do in a MySQL Cluster?
A MySQL Cluster has both a physical and logical organization, with computers being the physical elements. The logical or functional elements of a cluster are referred to as nodes, and a computer housing a cluster node is sometimes referred to as a cluster host. There are three types of nodes, each corresponding to a specific role within the cluster. These are:
Management node (MGM node): Provides management services for the cluster as a whole, including startup, shutdown, backups, and configuration data for the other nodes. The management node server is implemented as the application ndb_mgmd; the management client used to control MySQL Cluster via the MGM node is ndb_mgm.
Data node: Stores and replicates data. Data node functionality is handled by an instance of the NDB data node process ndbd.
SQL node: This is
simply an instance of MySQL Server
(mysqld) that is built with support
for the NDB Cluster
storage engine
and started with the --ndb-cluster
option to enable the engine.
28.10.6: With which operating systems can I use Cluster?
MySQL Cluster is supported on most Unix-like operating systems, including Linux, Mac OS X, Solaris, BSD, HP-UX, AIX, and IRIX, among others, as well as Novell Netware. Cluster is not supported for Windows at this time. However, we are working to add Cluster support for other platforms, including Windows, and our goal is to offer MySQL Cluster on all platforms for which MySQL itself is supported.
For more detailed information concerning the level of support which is offered for MySQL Cluster on various operating system versions, OS distributions, and hardware platforms, please refer to http://www.mysql.com/support/supportedplatforms/cluster.html.
28.10.7: What are the hardware requirements for running MySQL Cluster?
Cluster should run on any platform for which NDB-enabled binaries are available. Naturally, faster CPUs and more memory will improve performance, and 64-bit CPUs will likely be more effective than 32-bit processors. There must be sufficient memory on machines used for data nodes to hold each node's share of the database (see How much RAM do I Need? for more information). Nodes can communicate via a standard TCP/IP network and hardware. For SCI support, special networking hardware is required.
28.10.8: How much RAM do I need? Is it possible to use disk memory at all?
In MySQL-5.0, Cluster is in-memory only. This means that all table data (including indexes) is stored in RAM. Therefore, if your data takes up 1GB of space and you want to replicate it once in the cluster, you need 2GB of memory to do so (1 GB per replica). This is in addition to the memory required by the operating system and any applications running on the cluster computers.
If a data node's memory usage exceeds what is available in
RAM, then the system will attempt to use swap space up to
the limit set for DataMemory
. However,
this will at best result in severely degraded performance,
and may cause the node to be dropped due to slow response
time (missed heartbeats). We do not recommend on relying on
disk swapping in a production environment for this reason.
In any case, once the DataMemory
limit is
reached, any operations requiring additional memory (such as
inserts) will fail.
(We have implemented disk data storage for MySQL Cluster in MySQL 5.1, but we have no plans to add this capability in MySQL 5.0.)
You can use the following formula for obtaining a rough estimate of how much RAM is needed for each data node in the cluster:
(SizeofDatabase × NumberOfReplicas × 1.1 ) / NumberOfDataNodes
To calculate the memory requirements more exactly requires determining, for each table in the cluster database, the storage space required per row (see Section 10.5, “Data Type Storage Requirements”, for details), and multiplying this by the number of rows. You must also remember to account for any column indexes as follows:
Each primary key or hash index created for an
NDBCluster
table requires 21–25
bytes per record. These indexes use
IndexMemory
.
Each ordered index requires 10 bytes storage per record,
using DataMemory
.
Creating a primary key or unique index also creates an
ordered index, unless this index is created with
USING HASH
. In other words:
A primary key or unique index on a Cluster table normally takes up 31 to 35 bytes per record.
However, if the primary key or unique index is
created with USING HASH
, then it
requires only 21 to 25 bytes per record.
Note that creating MySQL Cluster tables with USING
HASH
for all primary keys and unique indexes will
generally cause table updates to run more quickly — in
some cases by a much as 20 to 30 percent faster than updates
on tables where USING HASH
was not used
in creating primary and unique keys. This is due to the fact
that less memory is required (because no ordered indexes are
created), and that less CPU must be utilized (because fewer
indexes must be read and possibly updated). However, it also
means that queries that could otherwise use range scans must
be satisfied by other means, which can result in slower
selects.
When calculating Cluster memory requirements, you may find
useful the ndb_size.pl
utility which is
available in recent MySQL 5.0 releases. This
Perl script connects to a current (non-Cluster) MySQL
database and creates a report on how much space that
database would require if it used the
NDBCluster
storage engine. For more
information, see
Section 16.10.14, “ndb_size.pl — NDBCluster Size Requirement Estimator”.
It is especially important to keep in mind that
every MySQL Cluster table must have a primary
key. The NDB
storage engine
creates a primary key automatically if none is defined, and
this primary key is created without USING
HASH
.
There is no easy way to determine exactly how much memory is
being used for storage of Cluster indexes at any given time;
however, warnings are written to the Cluster log when 80% of
available DataMemory
or
IndexMemory
is in use, and again when use
reaches 85%, 90%, and so on.
28.10.9: What filesystems can I use with MySQL Cluster? What about network filesystems or network shares?
Generally, any filesystem that is native to the host operating system should work well with MySQL Cluster. If you find that a given filesystem works particularly well (or not so especially well) with MySQL Cluster, we invite you to discuss your findings in the MySQL Cluster Forums.
We do not test MySQL Cluster with FAT
or
VFAT
filesystems on Linux. Because of
this, and due to the fact that these are not very useful for
any purpose other than sharing disk partitions between Linux
and Windows operating systems on multi-boot computers, we do
not recommend their use with MySQL Cluster.
MySQL Cluster is implemented as a shared-nothing solution; the idea behind this is that the failure of a single piece of hardware should not cause the failure of multiple cluster nodes, or possibly even the failure of the cluster as a whole. For this reason, the use of network shares or network filesystems is not supported for MySQL Cluster. This also applies to shared storage devices such as SANs.
28.10.10: Can I run MySQL Cluster nodes inside virtual machines (such as those created by VMWare, Parallels, or Xen)?
This is possible but not recommended for a production environment.
We have found that running MySQL Cluster processes inside a virtual machine can give rise to issues with timing and disk subsystems that have a strong negative impact on the operation of the cluster. The behavior of the cluster is often unpredictable in these cases.
If the issue can be reproduced outside the virtual environment, then we may be able to provide assistance. Otherwise, we cannot support it at this time.
28.10.11:
I'm trying to populate a Cluster database. The loading
process terminates prematurely and I get an error message
like this one:
ERROR 1114: The table 'my_cluster_table' is full
Why is this happening?
The cause is very likely to be that your setup does not
provide sufficient RAM for all table data and all indexes,
including the primary key required by the
NDB
storage engine and automatically
created in the event that the table definition does not
include the definition of a primary key.
It is also worth noting that all data nodes should have the same amount of RAM, since no data node in a cluster can use more memory than the least amount available to any individual data node. In other words, if there are four computers hosting Cluster data nodes, and three of these have 3GB of RAM available to store Cluster data while the remaining data node has only 1GB RAM, then each data node can devote only 1GB to clustering.
28.10.12: MySQL Cluster uses TCP/IP. Does this mean that I can run it over the Internet, with one or more nodes in remote locations?
It is very unlikely that a cluster would perform reliably under such conditions, as MySQL Cluster was designed and implemented with the assumption that it would be run under conditions guaranteeing dedicated high-speed connectivity such as that found in a LAN setting using 100 Mbps or gigabit Ethernet — preferably the latter. We neither test nor warrant its performance using anything slower than this.
Also, it is extremely important to keep in mind that communications between the nodes in a MySQL Cluster are not secure; they are neither encrypted nor safeguarded by any other protective mechanism. The most secure configuration for a cluster is in a private network behind a firewall, with no direct access to any Cluster data or management nodes from outside. (For SQL nodes, you should take the same precautions as you would with any other instance of the MySQL server.)
28.10.13: Do I have to learn a new programming or query language to use MySQL Cluster?
No. Although some specialized commands are used to manage and configure the cluster itself, only standard (My)SQL queries and commands are required for the following operations:
Creating, altering, and dropping tables
Inserting, updating, and deleting table data
Creating, changing, and dropping primary and unique indexes
Some specialized configuration parameters and files are required to set up a MySQL Cluster — see Section 16.3.4, “Configuration File”, for information about these.
A few simple commands are used in the MySQL Cluster management client for tasks such as starting and stopping cluster nodes. See Section 16.7.2, “Commands in the MySQL Cluster Management Client”.
28.10.14: How do I find out what an error or warning message means when using MySQL Cluster?
There are two ways in which this can be done:
28.10.15: Is MySQL Cluster transaction-safe? What isolation levels are supported?
Yes: For tables created with the
NDB
storage engine, transactions are
supported. In MySQL 5.0, Cluster supports only
the READ COMMITTED
transaction isolation
level.
28.10.16: What storage engines are supported by MySQL Cluster?
Clustering in MySQL is supported only by the
NDB
storage engine. That is, in order for
a table to be shared between nodes in a cluster, it must be
created using ENGINE=NDB
(or
ENGINE=NDBCLUSTER
, which is equivalent).
It is possible to create tables using other storage engines
(such as MyISAM
or
InnoDB
) on a MySQL server being used for
clustering, but these non-NDB
tables will
not participate in the
cluster; they are local to the individual MySQL server
instance on which they are created.
28.10.17: Which versions of the MySQL software support Cluster? Do I have to compile from source?
Cluster is supported in all server binaries in the
5.0 release series for operating systems on
which MySQL Cluster is available. See
Section 4.3.1, “mysqld — The MySQL Server”. You can determine whether your
server has NDB support using either the SHOW
VARIABLES LIKE 'have_%'
or SHOW
ENGINES
statement.
Linux users, please note that NDB
is
not included in the standard MySQL
server RPMs. Beginning with MySQL 5.0.4, there are separate
RPM packages for the NDB storage engine and accompanying
management and other tools; see the NDB RPM Downloads
section of the MySQL 5.0 Downloads page for
these. (Prior to 5.0.4, you had to use the
-max
binaries supplied as
.tar.gz
archives. This is still
possible, but is not required, so you can use your Linux
distribution's RPM manager if you prefer.)
You can also obtain NDB support by compiling MySQL from source, but it is not necessary to do so simply to use MySQL Cluster. To download the latest binary, RPM, or source distribution in the MySQL 5.0 series, visit http://dev.mysql.com/downloads/mysql/5.0.html.
28.10.18: In the event of a catastrophic failure — say, for instance, the whole city loses power and my UPS fails — would I lose all my data?
All committed transactions are logged. Therefore, although it is possible that some data could be lost in the event of a catastrophe, this should be quite limited. Data loss can be further reduced by minimizing the number of operations per transaction. (It is not a good idea to perform large numbers of operations per transaction in any case.)
28.10.19:
Is it possible to use FULLTEXT
indexes
with Cluster?
FULLTEXT
indexing is not supported by the
NDB
storage engine in MySQL
5.0, or by any storage engine other than
MyISAM
. We are working to add this
capability in a future release.
28.10.20: Can I run multiple nodes on a single computer?
It is possible but not advisable. One of the chief reasons to run a cluster is to provide redundancy. To enjoy the full benefits of this redundancy, each node should reside on a separate machine. If you place multiple nodes on a single machine and that machine fails, you lose all of those nodes. Given that MySQL Cluster can be run on commodity hardware loaded with a low-cost (or even no-cost) operating system, the expense of an extra machine or two is well worth it to safeguard mission-critical data. It also worth noting that the requirements for a cluster host running a management node are minimal. This task can be accomplished with a 200 MHz Pentium CPU and sufficient RAM for the operating system plus a small amount of overhead for the ndb_mgmd and ndb_mgm processes.
It is acceptable to run multiple cluster data nodes on a single host for learning about MySQL Cluster, or for testing purposes; however, this is not supported for production use.
28.10.21: Can I add nodes to a cluster without restarting it?
Not at present. A simple restart is all that is required for adding new MGM or SQL nodes to a Cluster. When adding data nodes the process is more complex, and requires the following steps:
Make a complete backup of all Cluster data.
Completely shut down the cluster and all cluster node processes.
Restart the cluster, using the
--initial
startup option.
Never use the --initial
when starting
ndbd except when necessary to clear
the data node filesystem. See
Section 16.6.5.1, “Command Options for ndbd”,
for information about when this is required.
Restore all cluster data from the backup.
In a future MySQL Cluster release series, we hope to implement a “hot” reconfiguration capability for MySQL Cluster to minimize (if not eliminate) the requirement for restarting the cluster when adding new nodes. However, this is not planned for MySQL 5.0.
28.10.22: Are there any limitations that I should be aware of when using MySQL Cluster?
Limitations on NDB
tables in MySQL
5.0 include:
Temporary tables are not supported; a CREATE
TEMPORARY TABLE
statement using
ENGINE=NDB
or
ENGINE=NDBCLUSTER
fails with an
error.
FULLTEXT
indexes and index prefixes
are not supported. Only complete columns may be indexed.
Spatial data types are not supported. See Chapter 17, Spatial Extensions.
Only complete rollbacks for transactions are supported. Partial rollbacks and rollbacks to save points are not supported.
The maximum number of attributes allowed per table is 128, and attribute names cannot be any longer than 31 characters. For each table, the maximum combined length of the table and database names is 122 characters.
The maximum size for a table row is 8 kilobytes, not
counting BLOB
values. There is no set
limit for the number of rows per table. Table size
limits depend on a number of factors, in particular on
the amount of RAM available to each data node.
The NDB
engine does not support
foreign key constraints. As with
MyISAM
tables, these are ignored.
For a complete listing of limitations in MySQL Cluster, see Section 16.12, “Known Limitations of MySQL Cluster”.
28.10.23: How do I import an existing MySQL database into a cluster?
You can import databases into MySQL Cluster much as you
would with any other version of MySQL. Other than the
limitations mentioned elsewhere in this FAQ and in
Section 16.12, “Known Limitations of MySQL Cluster”, the only other
special requirement is that any tables to be included in the
cluster must use the NDB
storage engine.
This means that the tables must be created with
ENGINE=NDB
or
ENGINE=NDBCLUSTER
.
It is also possible to convert existing tables using other
storage engines to NDB Cluster
using one
or more ALTER TABLE
statement, but this
requires an additional workaround. See
Section 16.12, “Known Limitations of MySQL Cluster”, for details.
28.10.24: How do cluster nodes communicate with one another?
Cluster nodes can communicate via any of three different protocols: TCP/IP, SHM (shared memory), and SCI (Scalable Coherent Interface). Where available, SHM is used by default between nodes residing on the same cluster host; however, this is considered experimental in MySQL 5.0. SCI is a high-speed (1 gigabit per second and higher), high-availability protocol used in building scalable multi-processor systems; it requires special hardware and drivers. See Section 16.11, “Using High-Speed Interconnects with MySQL Cluster”, for more about using SCI as a transport mechanism in MySQL Cluster.
28.10.25: What is an arbitrator?
If one or more nodes in a cluster fail, it is possible that not all cluster nodes will be able to “see” one another. In fact, it is possible that two sets of nodes might become isolated from one another in a network partitioning, also known as a “split brain” scenario. This type of situation is undesirable because each set of nodes tries to behave as though it is the entire cluster.
When cluster nodes go down, there are two possibilities. If more than 50% of the remaining nodes can communicate with each other, we have what is sometimes called a “majority rules” situation, and this set of nodes is considered to be the cluster. The arbitrator comes into play when there is an even number of nodes: in such cases, the set of nodes to which the arbitrator belongs is considered to be the cluster, and nodes not belonging to this set are shut down.
The preceding information is somewhat simplified. A more complete explanation taking into account node groups follows:
When all nodes in at least one node group are alive, network
partitioning is not an issue, because no one portion of the
cluster can form a functional cluster. The real problem
arises when no single node group has all its nodes alive, in
which case network partitioning (the
“split-brain” scenario) becomes possible. Then
an arbitrator is required. All cluster nodes recognize the
same node as the arbitrator, which is normally the
management server; however, it is possible to configure any
of the MySQL Servers in the cluster to act as the arbitrator
instead. The arbitrator accepts the first set of cluster
nodes to contact it, and tells the remaining set to shut
down. Arbitrator selection is controlled by the
ArbitrationRank
configuration parameter
for MySQL Server and management server nodes. (See
Section 16.3.4.4, “Defining the Management Server”, for
details.) It should also be noted that the role of
arbitrator does not in and of itself impose any heavy
demands upon the host so designated, and thus the arbitrator
host does not need to be particularly fast or to have extra
memory especially for this purpose.
28.10.26: What data types are supported by MySQL Cluster?
MySQL Cluster supports all of the usual MySQL data types,
with the exception of those associated with MySQL's spatial
extensions. (See Chapter 17, Spatial Extensions.) In
addition, there are some differences with regard to indexes
when used with NDB
tables.
MySQL Cluster tables (that is, tables created with
ENGINE=NDBCLUSTER
) have only
fixed-width rows. This means that (for example) each
record containing a VARCHAR(255)
column
will require space for 255 characters (as required for the
character set and collation being used for the table),
regardless of the actual number of characters stored
therein. This issue is expected to be fixed in a future
MySQL release series.
See Section 16.12, “Known Limitations of MySQL Cluster”, for more information about these issues.
28.10.27: How do I start and stop MySQL Cluster?
It is necessary to start each node in the cluster separately, in the following order:
Start the management node with the ndb_mgmd command.
Start each data node with the ndbd command.
Start each MySQL server (SQL node) using mysqld_safe --user=mysql &.
Each of these commands must be run from a system shell on
the machine housing the affected node. (You do not have to
be physically present at the machine — a remote login
shell can be used for this purpose.) You can verify that the
cluster is running by starting the MGM management client
ndb_mgm on the machine housing the MGM
node and issuing the SHOW
or ALL
STATUS
command.
To shut down a running cluster, issue the command
SHUTDOWN
in the MGM client.
Alternatively, you may enter the following command in a
system shell on the machine hosting the MGM node:
shell> ndb_mgm -e "SHUTDOWN"
(Note that the quotation marks are optional here; the
SHUTDOWN
command itself is not
case-sensitive.)
Either of these commands causes the ndb_mgm, ndb_mgm, and any ndbd processes to terminate gracefully. MySQL servers running as Cluster SQL nodes can be stopped using mysqladmin shutdown.
For more information, see Section 16.7.2, “Commands in the MySQL Cluster Management Client”, and Section 16.2.6, “Safe Shutdown and Restart”.
28.10.28: What happens to cluster data when the cluster is shut down?
The data that was held in memory by the cluster's data nodes is written to disk, and is reloaded into memory the next time that the cluster is started.
28.10.29: Is it helpful to have more than one management node for a cluster?
It can be helpful as a fail-safe. Only one MGM node controls the cluster at any given time, but it is possible to configure one MGM as primary, and one or more additional management nodes to take over in the event that the primary MGM node fails.
See Section 16.3.4, “Configuration File”, for information on how to configure MySQL Cluster management nodes.
28.10.30: Can I mix different kinds of hardware and operating systems in one MySQL Cluster?
Yes, so long as all machines and operating systems have the same “endianness” (all big-endian or all little-endian). It is also possible to use different MySQL Cluster releases on different nodes. However, we recommend this be done only as part of a rolling upgrade procedure (see Section 16.5.1, “Performing a Rolling Restart of the Cluster”).
28.10.31: Can I run two data nodes on a single host? Two SQL nodes?
Yes, it is possible to do this. In the case of multiple data nodes, it is advisable (but not required) for each node to use a different data directory. If you want to run multiple SQL nodes on one machine, each instance of mysqld must use a different TCP/IP port. However, running more than one cluster node of a given type per machine is not supported for production use.
28.10.32: Can I use hostnames with MySQL Cluster?
Yes, it is possible to use DNS and DHCP for cluster hosts. However, if your application requires “five nines” availability, we recommend using fixed IP addresses. Making communication between Cluster hosts dependent on services such as DNS and DHCP introduces additional points of failure, and the fewer of these, the better.
28.10.33: How do I handle MySQL users in a Cluster having multiple MySQL servers?
MySQL user accounts and privileges are not automatically propagated between different MySQL servers accessing the same MySQL Cluster. Therefore, you must make sure that these are copied between the SQL nodes yourself.
This set of Frequently Asked Questions derives from the experience of MySQL's Support and Development groups in handling many inquiries about CJK (Chinese-Japanese-Korean) issues.
Questions
28.11.1: What CJK character sets are available in MySQL?
28.11.2:
I have inserted CJK characters into my table. Why does
SELECT
display them as “?”
characters?
28.11.3: What problems should I be aware of when working with the Big5 Chinese character set?
28.11.4: Why do Japanese character set conversions fail?
28.11.5:
What should I do if I want to convert SJIS
81CA
to cp932
?
28.11.6:
How does MySQL represent the Yen (¥
)
sign?
28.11.7:
Do MySQL plan to make a separate character set where
5C
is the Yen sign, as at least one other
major DBMS does?
28.11.8: Of what issues should I be aware when working with Korean character sets in MySQL?
28.11.9: Why do I get Data truncated error messages?
28.11.10: Why does my GUI front end or browser not display CJK characters correctly in my application using Access, PHP, or another API?
28.11.11: I've upgraded to MySQL 5.0. How can I revert to behavior like that in MySQL 4.0 with regard to character sets?
28.11.12:
Why do some LIKE
and
FULLTEXT
searches with CJK characters
fail?
28.11.13:
How do I know whether character X
is available in all character sets?
28.11.14: Why don't CJK strings sort correctly in Unicode? (I)
28.11.15: Why don't CJK strings sort correctly in Unicode? (II)
28.11.16: Why are my supplementary characters rejected by MySQL?
28.11.17: Shouldn't it be “CJKV”?
28.11.18: Does MySQL allow CJK characters to be used in database and table names?
28.11.19: Where can I find translations of the MySQL Manual into Chinese, Japanese, and Korean?
28.11.20: Where can I get help with CJK and related issues in MySQL?
Questions and Answers
28.11.1: What CJK character sets are available in MySQL?
The list of CJK character sets may vary depending on your
MySQL version. For example, the eucjpms
character set was not supported prior to MySQL 5.0.3 (see
Section E.1.27, “Changes in MySQL 5.0.3 (23 March 2005: Beta)”). However, since the name of
the applicable language appears in the
DESCRIPTION
column for every entry in the
INFORMATION_SCHEMA.CHARACTER_SETS
table,
you can obtain a current list of all the non-Unicode CJK
character sets using this query:
mysql>SELECT CHARACTER_SET_NAME, DESCRIPTION
->FROM INFORMATION_SCHEMA.CHARACTER_SETS
->WHERE DESCRIPTION LIKE '%Chinese%'
->OR DESCRIPTION LIKE '%Japanese%'
->OR DESCRIPTION LIKE '%Korean%'
->ORDER BY CHARACTER_SET_NAME;
+--------------------+---------------------------+ | CHARACTER_SET_NAME | DESCRIPTION | +--------------------+---------------------------+ | big5 | Big5 Traditional Chinese | | cp932 | SJIS for Windows Japanese | | eucjpms | UJIS for Windows Japanese | | euckr | EUC-KR Korean | | gb2312 | GB2312 Simplified Chinese | | gbk | GBK Simplified Chinese | | sjis | Shift-JIS Japanese | | ujis | EUC-JP Japanese | +--------------------+---------------------------+ 8 rows in set (0.01 sec)
(See Section 21.9, “The INFORMATION_SCHEMA CHARACTER_SETS
Table”, for more
information.)
MySQL supports the two common variants of the
GB (Guojia
Biaozhun, or National
Standard, or Simplified
Chinese) character sets which are official in the
People's Republic of China: gb2312
and
gbk
. Sometimes people try to insert
gbk
characters into
gb2312
, and it works most of the time
because gbk
is a superset of
gb2312
— but eventually they try to
insert a rarer Chinese character and it doesn't work. (See
Bug#16072 for an example).
Here, we try to clarify exactly what characters are
legitimate in gb2312
or
gbk
, with reference to the official
documents. Please check these references before reporting
gb2312
or gbk
bugs.
For a complete listing of the
gb2312
characters, ordered
according to the gb2312_chinese_ci
collation:
gb2312
MySQL's gbk
is in reality
“Microsoft code page 936”. This differs
from the official gbk
for
characters A1A4
(middle dot),
A1AA
(em dash),
A6E0-A6F5
, and
A8BB-A8C0
. For a listing of the
differences, see
http://recode.progiciels-bpi.ca/showfile.html?name=dist/libiconv/gbk.h.
For a listing of gbk
/Unicode
mappings, see
http://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/CP936.TXT.
For MySQL's listing of gbk
characters, see
gbk.
28.11.2:
I have inserted CJK characters into my table. Why does
SELECT
display them as “?”
characters?
This problem is usually due to a setting in MySQL that doesn't match the settings for the application program or the operating system. Here are some common steps for correcting these types of issues:
Be certain of what MySQL version you are using.
Use the statement SELECT VERSION();
to determine this.
Make sure that the database is actually using the desired character set.
People often think that the client character set is
always the same as either the server character set or
the character set used for display purposes. However,
both of these are false assumptions. You can make sure
by checking the result of SHOW CREATE TABLE
or
— better — yet by using this statement:
tablename
SELECT character_set_name, collation_name FROM information_schema.columns WHERE table_schema = your_database_name AND table_name = your_table_name AND column_name = your_column_name;
Determine the hexadecimal value of the character or characters that are not being displayed correctly.
You can obtain this information for a column
column_name
in the table
table_name
using the
following query:
SELECT HEX(column_name
) FROMtable_name
;
3F
is the encoding for the
?
character; this means that
?
is the character actually stored in
the column. This most often happens because of a problem
converting a particular character from your client
character set to the target character set.
Make sure that a round trip possible —
that is, when you select
literal
(or
_introducer
hexadecimal-value
), you obtain
literal
as a
result.
For example, the Japanese
Katakana character
Pe
(ペ'
) exists in all CJK character
sets, and has the code point value (hexadecimal coding)
0x30da
. To test a round trip for this
character, use this query:
SELECT 'ペ' AS `ペ`; /* or SELECT _ucs2 0x30da; */
If the result is not also ペ
, then
the round trip has failed.
For bug reports regarding such failures, we might ask
you to follow up with SELECT
HEX('ペ');
. Then we can determine whether the
client encoding is correct.
Make sure that the problem is not with the browser or other application, rather than with MySQL.
Use the mysql client program (on Windows: mysql.exe) to accomplish this task. If mysql displays correctly but your application doesn't, then your problem is probably due to system settings.
To find out what your settings are, use the
SHOW VARIABLES
statement, whose
output should resemble what is shown here:
mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------------------+
| Variable_name | Value |
+--------------------------+----------------------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql/share/mysql/charsets/ |
+--------------------------+----------------------------------------+
8 rows in set (0.03 sec)
These are typical character-set settings for an
international-oriented client (notice the use of
utf8
Unicode) connected to a server
in the West (latin1
is a West Europe
character set and a default for MySQL).
Although Unicode (usually the utf8
variant on Unix, and the ucs2
variant
on Windows) is preferable to Latin, it's often not what
your operating system utilities support best. Many
Windows users find that a Microsoft character set, such
as cp932
for Japanese Windows, is
what's suitable.
If you cannot control the server settings, and you have
no idea what your underlying computer is, then try
changing to a common character set for the country that
you're in (euckr
= Korea;
gb2312
or gbk
=
People's Republic of China; big5
=
Taiwan; sjis
,
ujis
, cp932
, or
eucjpms
= Japan;
ucs2
or utf8
=
anywhere). Usually it is necessary to change only the
client and connection and results settings. There is a
simple statement which changes all three at once:
SET NAMES
. For example:
SET NAMES 'big5';
Once the setting is correct, you can make it permanent
by editing my.cnf
or
my.ini
. For example you might add
lines looking like these:
[mysqld] character-set-server=big5 [client] default-character-set=big5
It is also possible that there are issues with the API configuration setting being used in your application; see Why does my GUI front end or browser not display CJK characters correctly...? for more information.
28.11.3: What problems should I be aware of when working with the Big5 Chinese character set?
MySQL supports the Big5 character set which is common in
Hong Kong and Taiwan (Republic of China). MySQL's
big5
is in reality Microsoft code page
950, which is very similar to the original
big5
character set. We changed to this
character set starting with MySQL version 4.1.16 / 5.0.16
(as a result of Bug#12476). For example, the following
statements work in current versions of MySQL, but not in old
versions:
mysql>CREATE TABLE big5 (BIG5 CHAR(1) CHARACTER SET BIG5);
Query OK, 0 rows affected (0.13 sec) mysql>INSERT INTO big5 VALUES (0xf9dc);
Query OK, 1 row affected (0.00 sec) mysql>SELECT * FROM big5;
+------+ | big5 | +------+ | 嫺 | +------+ 1 row in set (0.02 sec)
A feature request for adding HKSCS
extensions has been filed. People who need this extension
may find the suggested patch for Bug#13577 to be of
interest.
28.11.4: Why do Japanese character set conversions fail?
MySQL supports the sjis
,
ujis
, cp932
, and
eucjpms
character sets, as well as
Unicode. A common need is to convert between character sets.
For example, there might be a Unix server (typically with
sjis
or ujis
) and a
Windows client (typically with cp932
).
In the following conversion table, the
ucs2
column represents the source, and
the sjis
, cp932
,
ujis
, and eucjpms
columns represent the destinations — that is, the last
4 columns provide the hexadecimal result when we use
CONVERT(ucs2)
or we assign a
ucs2
column containing the value to an
sjis
, cp932
,
ujis
, or eucjpms
column.
Character Name | ucs2 | sjis | cp932 | ujis | eucjpms |
---|---|---|---|---|---|
BROKEN BAR | 00A6 | 3F | 3F | 8FA2C3 | 3F |
FULLWIDTH BROKEN BAR | FFE4 | 3F | FA55 | 3F | 8FA2 |
YEN SIGN | 00A5 | 3F | 3F | 20 | 3F |
FULLWIDTH YEN SIGN | FFE5 | 818F | 818F | A1EF | 3F |
TILDE | 007E | 7E | 7E | 7E | 7E |
OVERLINE | 203E | 3F | 3F | 20 | 3F |
HORIZONTAL BAR | 2015 | 815C | 815C | A1BD | A1BD |
EM DASH | 2014 | 3F | 3F | 3F | 3F |
REVERSE SOLIDUS | 005C | 815F | 5C | 5C | 5C |
FULLWIDTH "" | FF3C | 3F | 815F | 3F | A1C0 |
WAVE DASH | 301C | 8160 | 3F | A1C1 | 3F |
FULLWIDTH TILDE | FF5E | 3F | 8160 | 3F | A1C1 |
DOUBLE VERTICAL LINE | 2016 | 8161 | 3F | A1C2 | 3F |
PARALLEL TO | 2225 | 3F | 8161 | 3F | A1C2 |
MINUS SIGN | 2212 | 817C | 3F | A1DD | 3F |
FULLWIDTH HYPHEN-MINUS | FF0D | 3F | 817C | 3F | A1DD |
CENT SIGN | 00A2 | 8191 | 3F | A1F1 | 3F |
FULLWIDTH CENT SIGN | FFE0 | 3F | 8191 | 3F | A1F1 |
POUND SIGN | 00A3 | 8192 | 3F | A1F2 | 3F |
FULLWIDTH POUND SIGN | FFE1 | 3F | 8192 | 3F | A1F2 |
NOT SIGN | 00AC | 81CA | 3F | A2CC | 3F |
FULLWIDTH NOT SIGN | FFE2 | 3F | 81CA | 3F | A2CC |
Now consider this portion of the table:
ucs2 | sjis | cp932 | |
---|---|---|---|
NOT SIGN | 00AC | 81CA | 3F |
FULLWIDTH NOT SIGN | FFE2 | 3F | 81CA |
This means that MySQL converts the NOT
SIGN
(Unicode U+00AC
) to
sjis
code point 0x81CA
and to cp932
code point
3F
. (3F
is the
question mark (“?”) — this is what is
always used when the conversion cannot be performed.
28.11.5:
What should I do if I want to convert SJIS
81CA
to cp932
?
Our answer is: “?”. There are serious
complaints about this: many people would prefer a
“loose” conversion, so that 81CA (NOT
SIGN)
in sjis
becomes
81CA (FULLWIDTH NOT SIGN)
in
cp932
. We are considering a change to
this behavior.
28.11.6:
How does MySQL represent the Yen (¥
)
sign?
A problem arises because some versions of Japanese character
sets (both sjis
and
euc
) treat 5C
as a
reverse solidus (\
— also known as a backslash), and others treat it as a
yen sign (¥
).
MySQL follows only one version of the JIS (Japanese
Industrial Standards) standard description. In MySQL,
5C
is always the reverse
solidus (\
).
28.11.7:
Do MySQL plan to make a separate character set where
5C
is the Yen sign, as at least one other
major DBMS does?
This is one possible solution to the Yen sign issue; however, this will not happen in MySQL 5.1 or 5.2.
28.11.8: Of what issues should I be aware when working with Korean character sets in MySQL?
In theory, while there have been several versions of the
euckr
(Extended Unix Code
Korea) character set, only one problem has been
noted.
We use the “ASCII” variant of EUC-KR, in which
the code point 0x5c
is REVERSE SOLIDUS,
that is \
, instead of the
“KS-Roman” variant of EUC-KR, in which the code
point 0x5c
is WON
SIGN
(₩
). This means that you
cannot convert Unicode U+20A9
to
euckr
:
mysql>SELECT
->CONVERT('₩' USING euckr) AS euckr,
->HEX(CONVERT('₩' USING euckr)) AS hexeuckr;
+-------+----------+ | euckr | hexeuckr | +-------+----------+ | ? | 3F | +-------+----------+ 1 row in set (0.00 sec)
MySQL's graphic Korean chart is here: euckr.
28.11.9: Why do I get Data truncated error messages?
For illustration, we'll create a table with one Unicode
(ucs2
) column and one Chinese
(gb2312
) column.
mysql>CREATE TABLE ch
->(ucs2 CHAR(3) CHARACTER SET ucs2,
->gb2312 CHAR(3) CHARACTER SET gb2312);
Query OK, 0 rows affected (0.05 sec)
We'll try to place the rare character 汌
in both columns.
mysql> INSERT INTO ch VALUES ('A汌B','A汌B');
Query OK, 1 row affected, 1 warning (0.00 sec)
Ah, there's a warning. Let's see what it is.
mysql> SHOW WARNINGS;
+---------+------+---------------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------------+
| Warning | 1265 | Data truncated for column 'gb2312' at row 1 |
+---------+------+---------------------------------------------+
1 row in set (0.00 sec)
So it's a warning about the gb2312
column
only.
mysql> SELECT ucs2,HEX(ucs2),gb2312,HEX(gb2312) FROM ch; +-------+--------------+--------+-------------+ | ucs2 | HEX(ucs2) | gb2312 | HEX(gb2312) | +-------+--------------+--------+-------------+ | A汌B | 00416C4C0042 | A?B | 413F42 | +-------+--------------+--------+-------------+ 1 row in set (0.00 sec)
There are several things that need explanation here.
The fact that it's a “warning” rather than an “error” is characteristic of MySQL. We like to try to do what we can, to get the best fit, rather than give up.
The 汌
character isn't in the
gb2312
character set. We described
that problem earlier.
Admittedly the message is misleading. We didn't “truncate” in this case, we replaced with a question mark. We've had a complaint about this message (See Bug#9337). But until we come up with something better, just accept that error/warning code 2165 can mean a variety of things.
With SQL_MODE=TRADITIONAL
, there
would be an error message, but instead of error 2165
you would see: ERROR 1406 (22001): Data too
long for column 'gb2312' at row 1
.
28.11.10: Why does my GUI front end or browser not display CJK characters correctly in my application using Access, PHP, or another API?
Obtain a direct connection to the server using the
mysql client (Windows:
mysql.exe), and try the same query there.
If mysql responds correctly, then the
trouble may be that your application interface requires
initialization. Use mysql to tell you
what character set or sets it uses with the statement
SHOW VARIABLES LIKE 'char%';
. If you are
using Access, then you are most likely connecting with
MyODBC. In this case, you should check
Section 24.1.4, “Connector/ODBC Configuration”. If, for instance,
you use big5
, you would enter
SET NAMES 'big5'
. (Note that no
;
is required in this case). If you are
using ASP, you might need to add SET
NAMES
in the code. Here is an example that has
worked in the past:
<% Session.CodePage=0 Dim strConnection Dim Conn strConnection="driver={MySQL ODBC 3.51 Driver};server=server
;uid=username
;" \ & "pwd=password
;database=database
;stmt=SET NAMES 'big5';" Set Conn = Server.CreateObject("ADODB.Connection") Conn.Open strConnection %>
In much the same way, if you are using any character set
other than latin1
with Connector/NET,
then you must specify the character set in the connection
string. See
Section 24.2.5.1, “Connecting to MySQL Using Connector/NET”, for more
information.
If you are using PHP, try this:
<?php $link = mysql_connect($host, $usr, $pwd); mysql_select_db($db); if( mysql_error() ) { print "Database ERROR: " . mysql_error(); } mysql_query("SET NAMES 'utf8'", $link); ?>
In this case, we used SET NAMES
to change
character_set_client
and
character_set_connection
and
character_set_results
.
We encourage the use of the newer mysqli
extension, rather than mysql
. Using
mysqli
, the previous example could be
rewritten as shown here:
<?php $link = new mysqli($host, $usr, $pwd, $db); if( mysqli_connect_errno() ) { printf("Connect failed: %s\n", mysqli_connect_error()); exit(); } $link->query("SET NAMES 'utf8'"); ?>
Another issue often encountered in PHP applications has to
do with assumptions made by the browser. Sometimes adding or
changing a <meta>
tag suffices to
correct the problem: for example, to insure that the user
agent interprets page content as UTF-8
,
you should include <meta
http-equiv="Content-Type" content="text/html;
charset=utf-8">
in the
<head>
of the HTML page.
If you are using Connector/J, see Section 24.4.4.4, “Using Character Sets and Unicode”.
28.11.11: I've upgraded to MySQL 5.0. How can I revert to behavior like that in MySQL 4.0 with regard to character sets?
In MySQL Version 4.0, there was a single “global” character set for both server and client, and the decision as to which character to use was made by the server administrator. This changed starting with MySQL Version 4.1. What happens now is a “handshake”, as described in Section 9.1.4, “Connection Character Sets and Collations”:
When a client connects, it sends to the server the name of the character set that it wants to use. The server uses the name to set the
character_set_client
,character_set_results
, andcharacter_set_connection
system variables. In effect, the server performs aSET NAMES
operation using the character set name.
The effect of this is that you cannot control the client
character set by starting mysqld with
--character-set-server=utf8
. However, some
of our Asian customers have said that they prefer the MySQL
4.0 behavior. To make it possible to retain this behavior,
we added a mysqld switch,
--character-set-client-handshake
, which can
be turned off with
--skip-character-set-client-handshake
. If
you start mysqld with
--skip-character-set-client-handshake
,
then, when a client connects, it sends to the server the
name of the character set that it wants to use —
however, the server ignores this request from the
client.
By way of example, suppose that your favorite server
character set is latin1
(unlikely in a
CJK area, but this is the default value). Suppose further
that the client uses utf8
because this is
what the client's operating system supports. Now, start the
server with latin1
as its default
character set:
mysqld --character-set-server=latin1
And then start the client with the default character set
utf8
:
mysql --default-character-set=utf8
The current settings can be seen by viewing the output of
SHOW VARIABLES
:
mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------------------+
| Variable_name | Value |
+--------------------------+----------------------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql/share/mysql/charsets/ |
+--------------------------+----------------------------------------+
8 rows in set (0.01 sec)
Now stop the client, and then stop the server using mysqladmin. Then start the server again, but this time tell it to skip the handshake like so:
mysqld --character-set-server=utf8 --skip-character-set-client-handshake
Start the client with utf8
once again as
the default character set, then display the current
settings:
mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------------------+
| Variable_name | Value |
+--------------------------+----------------------------------------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql/share/mysql/charsets/ |
+--------------------------+----------------------------------------+
8 rows in set (0.01 sec)
As you can see by comparing the differing results from
SHOW VARIABLES
, the server ignores the
client's initial settings if the
--skip-character-set-client-handshake
is
used.
28.11.12:
Why do some LIKE
and
FULLTEXT
searches with CJK characters
fail?
There is a very simple problem with
LIKE
searches on
BINARY
and BLOB
columns: we need to know the end of a character. With
multi-byte character sets, different characters might have
different octet lengths. For example, in
utf8
, A
requires one
byte but ペ
requires three bytes, as
shown here:
+-------------------------+---------------------------+ | OCTET_LENGTH(_utf8 'A') | OCTET_LENGTH(_utf8 'ペ') | +-------------------------+---------------------------+ | 1 | 3 | +-------------------------+---------------------------+ 1 row in set (0.00 sec)
If we don't know where the first character ends, then we
don't know where the second character begins, in which case
even very simple searches such as
LIKE
'_A%'
fail. The solution is to use a regular CJK
character set in the first place, or to convert to a CJK
character set before comparing.
This is one reason why MySQL cannot allow encodings of nonexistent characters. If it is not strict about rejecting bad input, then it has no way of knowing where characters end.
For FULLTEXT
searches, we need to know
where words begin and end. With Western languages, this is
rarely a problem because most (if not all) of these use an
easy-to-identify word boundary — the space character.
However, this is not usually the case with Asian writing. We
could use arbitrary halfway measures, like assuming that all
Han characters represent words, or (for Japanese) depending
on changes from Katakana to Hiragana due to grammatical
endings. However, the only sure solution requires a
comprehensive word list, which means that we would have to
include a dictionary in the server for each Asian language
supported. This is simply not feasible.
28.11.13:
How do I know whether character X
is available in all character sets?
The majority of simplified Chinese and basic non-halfwidth
Japanese Kana characters
appear in all CJK character sets. This stored procedure
accepts a UCS-2
Unicode character,
converts it to all other character sets, and displays the
results in hexadecimal.
DELIMITER // CREATE PROCEDURE p_convert(ucs2_char CHAR(1) CHARACTER SET ucs2) BEGIN CREATE TABLE tj (ucs2 CHAR(1) character set ucs2, utf8 CHAR(1) character set utf8, big5 CHAR(1) character set big5, cp932 CHAR(1) character set cp932, eucjpms CHAR(1) character set eucjpms, euckr CHAR(1) character set euckr, gb2312 CHAR(1) character set gb2312, gbk CHAR(1) character set gbk, sjis CHAR(1) character set sjis, ujis CHAR(1) character set ujis); INSERT INTO tj (ucs2) VALUES (ucs2_char); UPDATE tj SET utf8=ucs2, big5=ucs2, cp932=ucs2, eucjpms=ucs2, euckr=ucs2, gb2312=ucs2, gbk=ucs2, sjis=ucs2, ujis=ucs2; /* If there's a conversion problem, UPDATE will produce a warning. */ SELECT hex(ucs2) AS ucs2, hex(utf8) AS utf8, hex(big5) AS big5, hex(cp932) AS cp932, hex(eucjpms) AS eucjpms, hex(euckr) AS euckr, hex(gb2312) AS gb2312, hex(gbk) AS gbk, hex(sjis) AS sjis, hex(ujis) AS ujis FROM tj; DROP TABLE tj; END//
The input can be any single ucs2
character, or it can be the code point value (hexadecimal
representation) of that character. For example, from
Unicode's list of ucs2
encodings and
names
(http://www.unicode.org/Public/UNIDATA/UnicodeData.txt),
we know that the Katakana
character Pe appears in all
CJK character sets, and that its code point value is
0x30da
. If we use this value as the
argument to p_convert()
, the result is as
shown here:
mysql> CALL p_convert(0x30da)//
+------+--------+------+-------+---------+-------+--------+------+------+------+
| ucs2 | utf8 | big5 | cp932 | eucjpms | euckr | gb2312 | gbk | sjis | ujis |
+------+--------+------+-------+---------+-------+--------+------+------+------+
| 30DA | E3839A | C772 | 8379 | A5DA | ABDA | A5DA | A5DA | 8379 | A5DA |
+------+--------+------+-------+---------+-------+--------+------+------+------+
1 row in set (0.04 sec)
Since none of the column values is 3F
— that is, the question mark character
(?
) — we know that every conversion
worked.
28.11.14: Why don't CJK strings sort correctly in Unicode? (I)
Sometimes people observe that the result of a
utf8_unicode_ci
or
ucs2_unicode_ci
search, or of an
ORDER BY
sort is not what they think a
native would expect. Although we never rule out the
possibility that there is a bug, we have found in the past
that many people do not read correctly the standard table of
weights for the Unicode Collation Algorithm. MySQL uses the
table found at
http://www.unicode.org/Public/UCA/4.0.0/allkeys-4.0.0.txt.
This is not the first table you will find by navigating from
the unicode.org
home page, because MySQL
uses the older 4.0.0 “allkeys” table, rather
than the more recent 4.1.0 table. This is because we are
very wary about changing ordering which affects indexes,
lest we bring about situations such as that reported in Bug#16526, illustrated as follows:
mysql<CREATE TABLE tj (s1 CHAR(1) CHARACTER SET utf8 COLLATE utf8_unicode_ci);
Query OK, 0 rows affected (0.05 sec) mysql>INSERT INTO tj VALUES ('が'),('か');
Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql>SELECT * FROM tj WHERE s1 = 'か';
+------+ | s1 | +------+ | が | | か | +------+ 2 rows in set (0.00 sec)
The character in the first result row is not the one that we
searched for. Why did MySQL retrieve it? First we look for
the Unicode code point value, which is possible by reading
the hexadecimal number for the ucs2
version of the characters:
mysql> SELECT s1, HEX(CONVERT(s1 USING ucs2)) FROM tj;
+------+-----------------------------+
| s1 | HEX(CONVERT(s1 USING ucs2)) |
+------+-----------------------------+
| が | 304C |
| か | 304B |
+------+-----------------------------+
2 rows in set (0.03 sec)
Now we search for 304B
and
304C
in the 4.0.0
allkeys
table, and find these lines:
304B ; [.1E57.0020.000E.304B] # HIRAGANA LETTER KA 304C ; [.1E57.0020.000E.304B][.0000.0140.0002.3099] # HIRAGANA LETTER GA; QQCM
The official Unicode names (following the “#”
mark) tell us the Japanese syllabary (Hiragana), the
informal classification (letter, digit, or punctuation
mark), and the Western identifier (KA
or
GA
, which happen to be voiced and
unvoiced components of the same letter pair). More
importantly, the primary weight (the
first hexadecimal number inside the square brackets) is
1E57
on both lines. For comparisons in
both searching and sorting, MySQL pays attention to the
primary weight only, ignoring all the other numbers. This
means that we are sorting が
and
か
correctly according to the Unicode
specification. If we wanted to distinguish them, we'd have
to use a non-UCA (Unicode Collation Algorithm) collation
(utf8_bin
or
utf8_general_ci
), or to compare the
HEX()
values, or use ORDER BY
CONVERT(s1 USING sjis)
. Being correct
“according to Unicode” isn't enough, of course:
the person who submitted the bug was equally correct. We
plan to add another collation for Japanese according to the
JIS X 4061 standard, in which voiced/unvoiced letter pairs
like KA
/GA
are
distinguishable for ordering purposes.
28.11.15: Why don't CJK strings sort correctly in Unicode? (II)
If you are using Unicode (ucs2
or
utf8
), and you know what the Unicode sort
order is (see Section A.11, “MySQL 5.0 FAQ — MySQL Chinese, Japanese, and Korean
Character Sets”), but MySQL still
seems to sort your table incorrectly, then you should first
verify the table character set:
mysql> SHOW CREATE TABLE t\G
******************** 1. row ******************
Table: t
Create Table: CREATE TABLE `t` (
`s1` char(1) CHARACTER SET ucs2 DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
Since the character set appears to be correct, let's see
what information the
INFORMATION_SCHEMA.COLUMNS
table can
provide about this column:
mysql>SELECT COLUMN_NAME, CHARACTER_SET_NAME, COLLATION_NAME
->FROM INFORMATION_SCHEMA.COLUMNS
->WHERE COLUMN_NAME = 's1'
->AND TABLE_NAME = 't';
+-------------+--------------------+-----------------+ | COLUMN_NAME | CHARACTER_SET_NAME | COLLATION_NAME | +-------------+--------------------+-----------------+ | s1 | ucs2 | ucs2_general_ci | +-------------+--------------------+-----------------+ 1 row in set (0.01 sec)
(See Section 21.3, “The INFORMATION_SCHEMA COLUMNS
Table”, for more information.)
You can see that the collation is
ucs2_general_ci
instead of
ucs2_unicode_ci
. The reason why this is
so can be found using SHOW CHARSET
, as
shown here:
mysql> SHOW CHARSET LIKE 'ucs2%';
+---------+---------------+-------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+---------------+-------------------+--------+
| ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 |
+---------+---------------+-------------------+--------+
1 row in set (0.00 sec)
For ucs2
and utf8
, the
default collation is “general”. To specify a
Unicode collation, use COLLATE
ucs2_unicode_ci
.
28.11.16: Why are my supplementary characters rejected by MySQL?
Before MySQL 6.0.4, MySQL does not support supplementary
characters — that is, characters which need more than
3 bytes — for UTF-8
. We support
only what Unicode calls the Basic Multilingual
Plane / Plane 0. Only a few very rare Han
characters are supplementary; support for them is uncommon.
This has led to reports such as that found in Bug#12600,
which we rejected as “not a bug”. With
utf8
, we must truncate an input string
when we encounter bytes that we don't understand. Otherwise,
we wouldn't know how long the bad multi-byte character is.
One possible workaround is to use ucs2
instead of utf8
, in which case the
“bad” characters are changed to question marks;
however, no truncation takes place. You can also change the
data type to BLOB
or
BINARY
, which perform no validity
checking.
As of MySQL 6.0.4, Unicode support is extended to include
supplementary characters by means of additional Unicode
character sets: utf16
,
utf32
, and 4-byte
utf8
. These character sets support
supplementary Unicode characters outside the Basic
Multilingual Plane (BMP).
28.11.17: Shouldn't it be “CJKV”?
No. The term “CJKV” (Chinese Japanese Korean Vietnamese) refers to Vietnamese character sets which contain Han (originally Chinese) characters. MySQL has no plan to support the old Vietnamese script using Han characters. MySQL does of course support the modern Vietnamese script with Western characters.
Bug#4745 is a request for a specialized Vietnamese collation, which we might add in the future if there is sufficient demand for it.
28.11.18: Does MySQL allow CJK characters to be used in database and table names?
This issue is fixed in MySQL 5.1, by automatically rewriting the names of the corresponding directories and files.
For example, if you create a database named
楮
on a server whose operating system
does not support CJK in directory names, MySQL creates a
directory named @0w@00a5@00ae
. which is
just a fancy way of encoding E6A5AE
— that is, the Unicode hexadecimal representation for
the 楮
character. However, if you run a
SHOW DATABASES
statement, you can see
that the database is listed as 楮
.
28.11.19: Where can I find translations of the MySQL Manual into Chinese, Japanese, and Korean?
A Simplified Chinese version of the Manual, current for MySQL 5.1.12, can be found at http://dev.mysql.com/doc/. The Japanese translation of the MySQL 4.1 manual can be downloaded from http://dev.mysql.com/doc/.
28.11.20: Where can I get help with CJK and related issues in MySQL?
The following resources are available:
A listing of MySQL user groups can be found at http://dev.mysql.com/user-groups/.
You can contact a sales engineer at the MySQL KK Japan office using any of the following:
Tel: +81(0)3-5326-3133 Fax: +81(0)3-5326-3001 Email: dsaito@mysql.com
View feature requests relating to character set issues at http://tinyurl.com/y6xcuf.
Visit the MySQL Character Sets, Collation, Unicode Forum. We are also in the process of adding foreign-language forums at http://forums.mysql.com/.
For common questions, issues, and answers relating to the MySQL Connectors and other APIs, see the following areas of the Manual:
For answers to common queries and question regarding Replication within MySQL, see Section 15.3.4, “Replication FAQ”.
In the following section, we provide answers to questions that are most frequently asked about Distributed Replicated Block Device (DRBD).
Questions
28.14.1.1: What is DRBD?
28.14.1.2: What are “Block Devices”?
28.14.1.3: How is DRBD licensed?
28.14.1.4: Where can I download DRBD?
28.14.1.5: If I find a bug in DRBD, to whom do I submit the issue?
28.14.1.6: Where can I get more technical and business information concerning MySQL and DRBD?
Questions and Answers
DRBD is an acronym for Distributed Replicated Block Device. DRBD is an open source Linux kernel block device which leverages synchronous replication to achieve a consistent view of data between two systems, typically an Active and Passive system. DRBD currently supports all the major flavors of Linux and comes bundled in several major Linux distributions. The DRBD project is maintained by LINBIT.
28.14.1.2: What are “Block Devices”?
Block devices are the type of device used to represent storage in the Linux Kernel. All physical disk devices present a “block device” interface. Additionally, virtual disk systems like LVM or DRBD present a “block device” interface. In this way, the file system or other software that might want to access a disk device can be used with any number of real or virtual devices without having to know anything about their underlying implementation details.
28.14.1.3: How is DRBD licensed?
DRBD is licensed under the GPL.
28.14.1.4: Where can I download DRBD?
Please see http://www.drbd.org/download.html
28.14.1.5: If I find a bug in DRBD, to whom do I submit the issue?
Bug reports should be submitted to the DRBD mailing list. Please see: http://lists.linbit.com/ .
28.14.1.6: Where can I get more technical and business information concerning MySQL and DRBD?
Please visit: http://mysql.com/drbd/
In the following section, we provide answers to questions that are most frequently asked about Linux Heartbeat.
Questions
28.14.2.1: What is Linux Heartbeat?
28.14.2.2: How is Linux Heartbeat licensed?
28.14.2.3: Where can I download Linux Heartbeat?
28.14.2.4: If I find a bug with Linux Heartbeat, to whom do I submit the issue?
Questions and Answers
28.14.2.1: What is Linux Heartbeat?
The Linux-HA project (http://www.linux-ha.org/) offers a high availability solution commonly referred to as Linux Heartbeat. Linux Heartbeat ships as part of several Linux distributions, as well as within several embedded high availability systems. This solution can also be used for other applications besides databases servers, such as mail servers, web servers, file servers, and DNS servers.
Linux Heartbeat implements a heartbeat-protocol. A heartbeat-protocol means that messages are sent at regular intervals between two or more nodes. If a message is not received from a node within a given interval, then it is assumed the node has failed and some type of failover or recovery action is required. Linux Heartbeat is typically configured to send these heartbeat messages over standard Ethernet interfaces, but it does also support other methods, such as serial-line links.
28.14.2.2: How is Linux Heartbeat licensed?
Linux Heartbeat is licensed under the GPL.
28.14.2.3: Where can I download Linux Heartbeat?
Please see http://linux-ha.org/download/index.html.
28.14.2.4: If I find a bug with Linux Heartbeat, to whom do I submit the issue?
Bug reports should be submitted to http://www.linux-ha.org/ClusterResourceManager/BugReports.
In the following section, we provide answers to questions that are most frequently asked about DRBD Architecture.
Questions
28.14.3.1: Is an Active/Active option available for MySQL with DRBD?
28.14.3.2: What MySQL storage engines are supported with DRBD?
28.14.3.3: How long does a failover take?
28.14.3.4: How long does it take to resynchronize data after a failure?
28.14.3.5: Are there any situations where you shouldn't use DRBD?
28.14.3.6: Are there any limitations to DRBD?
Questions and Answers
28.14.3.1: Is an Active/Active option available for MySQL with DRBD?
Currently, MySQL does not support Active/Active configurations using DRBD “out of the box”.
28.14.3.2: What MySQL storage engines are supported with DRBD?
All of the MySQL transactional storage engines are supported by DRBD, including InnoDB and Falcon. For archived or read-only data, MyISAM or Archive can also be used.
28.14.3.3: How long does a failover take?
Failover time is dependent on many things, some of which are configurable. After activating the passive host, MySQL will have to start and run a normal recovery process. If the InnoDB log files have been configured to a large size and there was heavy write traffic, this may take a reasonably long period of time. However, under normal circumstances, failover tends to take less than a minute.
28.14.3.4: How long does it take to resynchronize data after a failure?
Resynchronization time depends on how long the two machines are out of communication and how much data was written during that period of time. Resynchronization time is a function of data to be synced, network speed and disk speed. DRBD maintains a bitmap of changed blocks on the primary machine, so only those blocks that have changed will need to be transferred.
28.14.3.5: Are there any situations where you shouldn't use DRBD?
See When Not To Use DRBD.
In the following section, we provide answers to questions that are most frequently asked about MySQL Replication Scale-out.
Questions
28.14.4.1: What is the difference between MySQL Cluster and DRBD?
28.14.4.2: What is the difference between MySQL Replication and DRBD?
28.14.4.3: How can I combine MySQL Replication scale-out with DRBD?
Questions and Answers
28.14.4.1: What is the difference between MySQL Cluster and DRBD?
Both MySQL Cluster and DRBD replicate data synchronously. MySQL Cluster leverages a shared-nothing storage architecture in which the cluster can be architected beyond an Active/Passive configuration. DRBD operates at a much lower level within the “stack”, at the disk I/O level. For a comparison of various high availability features between these two options, please refer to Chapter 14, High Availability and Scalability.
28.14.4.2: What is the difference between MySQL Replication and DRBD?
MySQL Replication replicates data asynchronously while DRBD replicates data synchronously. Also, MySQL Replication replicates MySQL statements, while DRBD replicates the underlying block device that stores the MySQL data files. For a comparison of various high availability features between these two options, please refer to the high availability comparison grid, Chapter 14, High Availability and Scalability.
28.14.4.3: How can I combine MySQL Replication scale-out with DRBD?
MySQL Replication is typically deployed in a Master to many Slaves configuration. In this configuration, having many Slaves provides read scalability. DRBD is used to provide high-availability for the Master MySQL Server in an Active/Passive configuration. This provides for automatic failover, safeguards against data loss, and automatically synchronizes the failed MySQL Master after a failover.
The most likely scenario in which MySQL Replication scale-out can be leveraged with DRBD is in the form of attaching replicated MySQL “read-slaves” off of the Active-Master MySQL Server, shown in Figure A.1, “Active-Master MySQL server”. Since DRBD replicates an entire block device, master information such as the binary logs are also replicated. In this way, all of the slaves can attach to the Virtual IP Address managed by Linux Heartbeat. In the event of a failure, the asynchronous nature of MySQL Replication allows the slaves to continue with the new Active machine as their master with no intervention needed.
In the following section, we provide answers to questions that are most frequently asked about DRBD and file systems.
Questions
28.14.5.1: Can XFS be used with DRBD?
Questions and Answers
28.14.5.1: Can XFS be used with DRBD?
Yes. XFS uses dynamic block size, thus DRBD 0.7 or later is needed.
In the following section, we provide answers to questions that are most frequently asked about DRBD and LVM.
Questions
28.14.6.1: Can I use DRBD on top of LVM?
28.14.6.2: Can I use LVM on top of DRBD?
28.14.6.3: Can I use DRBD on top of LVM while at the same time running LVM on top of that DRBD?
Questions and Answers
28.14.6.1: Can I use DRBD on top of LVM?
Yes, DRBD supports on-line resizing. If you enlarge your logical volume that acts as a backing device for DRBD, you can enlarge DRBD itself too, and of course your file system if it supports resizing.
28.14.6.2: Can I use LVM on top of DRBD?
Yes, you can use DRBD as a Physical Volume (PV) for LVM.
Depending on the default LVM configuration shipped with
your distribution, you may need to add the
/dev/drbd*
device files to the
filter
option in your
lvm.conf
so LVM scans your DRBDs for
PV signatures.
28.14.6.3: Can I use DRBD on top of LVM while at the same time running LVM on top of that DRBD?
This requires careful tuning of your LVM configuration to avoid duplicate PV scans, but yes, it is possible.
In the following section, we provide answers to questions that are most frequently asked about DRBD and virtualization.
Questions
28.14.7.1: Can I use DRBD with OpenVZ?
28.14.7.2: Can I use DRBD with Xen and/or KVM?
Questions and Answers
28.14.7.1: Can I use DRBD with OpenVZ?
See http://wiki.openvz.org/HA_cluster_with_DRBD_and_Heartbeat.
28.14.7.2: Can I use DRBD with Xen and/or KVM?
Yes. If you are looking for professional consultancy or expert commercial support for Xen- or KVM-based virtualization clusters with DRBD, contact LINBIT (http://www.linbit.com).
In the following section, we provide answers to questions that are most frequently asked about DRBD and security.
Questions
28.14.8.1: Can I encrypt/compress the exchanged data?
28.14.8.2: Does DRBD do mutual node authentication?
Questions and Answers
28.14.8.1: Can I encrypt/compress the exchanged data?
Yes. But there is no option within DRBD to allow for this. You’ll need to leverage a VPN and the network layer should do the rest.
28.14.8.2: Does DRBD do mutual node authentication?
Yes, starting with DRBD 8 shared-secret mutual node authentication is supported.
In the following section, we provide answers to questions that are most frequently asked about DRBD and System Requirements.
Questions
28.14.9.1: What other packages besides DRBD are required?
28.14.9.2: How many machines are required to set up DRBD?
28.14.9.3: Does DRBD only run on Linux?
Questions and Answers
28.14.9.1: What other packages besides DRBD are required?
When using pre-built binary packages, none except a
matching kernel, plus packages for
glibc
and your favorite shell. When
compiling DRBD from source additional prerequisite
packages may be required. They include but are not limited
to:
glib-devel
openssl
devel
libgcrypt-devel
glib2-devel
pkgconfig
ncurses-devel
rpm-build
rpm-devel
redhat-rpm-config
gcc
gcc-c++
bison
flex
gnutls-devel
lm_sensors-devel
net-snmp-devel
python-devel
bzip2-devel
libselinux-devel
perl-DBI
libnet
Pre-built x86 and x86_64 packages for specific kernel versions are available with a support subscription from LINBIT. Please note that if the kernel is upgraded, DRBD must be as well.
28.14.9.2: How many machines are required to set up DRBD?
Two machines are required to achieve the minimum degree of high availability. Although at any one given point in time one will be primary and one will be secondary, it is better to consider the machines as part of a mirrored pair without a “natural” primary machine.
28.14.9.3: Does DRBD only run on Linux?
DRBD is a Linux Kernel Module, and can work with many popular Linux distributions. DRBD is currently not available for non-Linux operating systems.
In the following section, we provide answers to questions that are most frequently asked about DRBD and resources.
Questions
28.14.10.1: Does MySQL offer professional consulting to help with designing a DRBD system?
28.14.10.2: Does MySQL offer support for DRBD and Linux Heartbeat from MySQL?
28.14.10.3: Are pre-built binaries or RPMs available?
28.14.10.4: Does MySQL have documentation to help me with the installation and configuration of DRBD and Linux Heartbeat?
28.14.10.5: Is there a dedicated discussion forum for MySQL High-Availability?
28.14.10.6: Where can I get more information about MySQL for DRBD?
Questions and Answers
28.14.10.1: Does MySQL offer professional consulting to help with designing a DRBD system?
Yes. MySQL offers consulting for the design, installation, configuration, and monitoring of high availability DRBD. For more information concerning a High Availability Jumpstart, please see: http://www.mysql.com/consulting/packaged/scaleout.html.
28.14.10.2: Does MySQL offer support for DRBD and Linux Heartbeat from MySQL?
Yes. Support for DRBD is available with an add-on subscription to MySQL Enterprise called “DRBD for MySQL”. For more information about support options for DRBD see: http://mysql.com/products/enterprise/features.html.
For the list of supported Linux distributions, please see: http://www.mysql.com/support/supportedplatforms/enterprise.html.
DRBD is only available on Linux. DRBD is not available on Windows, MacOS, Solaris, HPUX, AIX, FreeBSD, or other non-Linux platforms.
28.14.10.3: Are pre-built binaries or RPMs available?
Yes. “DRBD for MySQL” is an add-on subscription to MySQL Enterprise, which provides pre-built binaries for DRBD. For more information see: http://mysql.com/products/enterprise/features.html.
28.14.10.4: Does MySQL have documentation to help me with the installation and configuration of DRBD and Linux Heartbeat?
See Section 14.1, “Using MySQL with DRBD for High Availability”.
28.14.10.5: Is there a dedicated discussion forum for MySQL High-Availability?
Yes, http://forums.mysql.com/list.php?144.
28.14.10.6: Where can I get more information about MySQL for DRBD?
For more information about MySQL for DRBD, including a technical white paper please see: DRBD for MySQL High Availability.