<< Back to man.ChinaUnix.net

Go to the first, previous, next, last section, table of contents.


6 Replication in MySQL

Replication capabilities allowing the databases on one MySQL server to be duplicated on another were introduced in MySQL 3.23.15. This chapter describes the various replication features provided by MySQL. It introduces replication concepts, shows how to set up replication servers, and serves as a reference to the available replication options. It also provides a list of frequently asked questions (with answers), and troubleshooting advice for solving problems.

For a description of the syntax of replication-related SQL statements, see section 13.6 Replication Statements.

We suggest that you visit our Web site at http://www.mysql.com often and read updates to this chapter. Replication is constantly being improved, and we update the manual frequently with the most current information.

6.1 Introduction to Replication

MySQL 3.23.15 and up features support for one-way replication. One server acts as the master, while one or more other servers act as slaves. The master server writes updates to its binary log files, and maintains an index of the files to keep track of log rotation. These logs serve as a record of updates to be sent to slave servers. When a slave server connects to the master server, it informs the master of its last position within the logs since the last successfully propagated update. The slave catches up any updates that have occurred since then, and then blocks and waits for the master to notify it of new updates.

A slave server can also serve as a master if you want to set up chained replication servers.

Note that when you are using replication, all updates to the tables that are replicated should be performed on the master server. Otherwise, you must always be careful to avoid conflicts between updates that users make to tables on the master and updates that they make to tables on the slave.

One-way replication has benefits for robustness, speed, and system administration:

6.2 Replication Implementation Overview

MySQL replication is based on the master server keeping track of all changes to your databases (updates, deletes, and so on) in the binary logs. Therefore, to use replication, you must enable binary logging on the master server. See section 5.9.4 The Binary Log.

Each slave server receives from the master the saved updates that the master has recorded in its binary log, so that the slave can execute the same updates on its copy of the data.

It is very important to realize that the binary log is simply a record starting from the fixed point in time at which you enable binary logging. Any slaves that you set up will need copies of the databases on your master as they existed at the moment you enabled binary logging on the master. If you start your slaves with databases that are not the same as what was on the master when the binary log was started, your slaves may fail.

One way to copy the master's data to the slave is to use the LOAD DATA FROM MASTER statement. Be aware that LOAD DATA FROM MASTER is available only as of MySQL 4.0.0 and currently works only if all the tables on the master are MyISAM type. Also, this statement acquires a global read lock, so no updates on the master are possible while the tables are being transferred to the slave. When we implement lock-free hot table backup (in MySQL 5.0), this global read lock will no longer be necessary.

Due to these limitations, we recommend that at this point you use LOAD DATA FROM MASTER only if the dataset on the master is relatively small, or if a prolonged read lock on the master is acceptable. While the actual speed of LOAD DATA FROM MASTER may vary from system to system, a good rule of thumb for how long it will take is 1 second per 1MB of data. That is only a rough estimate, but you should get close to it if both master and slave are equivalent to 700MHz Pentium performance and are connected through a 100MBit/s network.

After the slave has been set up with a copy of the master's data, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect periodically until it is able to reconnect and resume listening for updates. The retry interval is controlled by the --master-connect-retry option. The default is 60 seconds.

Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up to date at any given time.

6.3 Replication Implementation Details

MySQL replication capabilities are implemented using three threads (one on the master server and two on the slave). When START SLAVE is issued, the slave creates an I/O thread. The I/O thread connects to the master and asks it to send the statements recorded in its binary logs. The master creates a thread to send the binary log contents to the slave. This thread can be identified as the Binlog Dump thread in the output of SHOW PROCESSLIST on the master. The slave I/O thread reads what the master Binlog Dump thread sends and simply copies it to some local files in the slave's data directory called relay logs. The third thread is the SQL thread, which the slave creates to read the relay logs and execute the updates they contain.

In the preceding description, there are three threads per slave. For a master that has multiple slaves, it creates one thread for each currently connected slave, and each slave has its own I/O and SQL threads.

For versions of MySQL before 4.0.2, replication involves only two threads (one on the master and one on the slave). The slave I/O and SQL threads are combined as a single thread, and no relay log files are used.

The advantage of using two slave threads is that statement reading and execution are separated into two independent tasks. The task of reading statements is not slowed down if statement execution is slow. For example, if the slave server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the master when the slave starts, even if the SQL thread lags far behind and may take hours to catch up. If the slave stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the statements is locally stored in the slave's relay logs for execution when next the slave starts. This allows the binary logs to be purged on the master, because it no longer need wait for the slave to fetch their contents.

The SHOW PROCESSLIST statement provides information that tells you what is happening on the master and on the slave regarding replication.

The following example illustrates how the three threads show up in SHOW PROCESSLIST. The output format is that used by SHOW PROCESSLIST as of MySQL version 4.0.15, when the content of the State column was changed to be more meaningful compared to earlier versions.

On the master server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 2
   User: root
   Host: localhost:32931
     db: NULL
Command: Binlog Dump
   Time: 94
  State: Has sent all binlog to slave; waiting for binlog to
         be updated
   Info: NULL

Here, thread 2 is a replication thread for a connected slave. The information indicates that all outstanding updates have been sent to the slave and that the master is waiting for more updates to occur.

On the slave server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 10
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Waiting for master to send event
   Info: NULL
*************************** 2. row ***************************
     Id: 11
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Has read all relay log; waiting for the slave I/O
         thread to update it
   Info: NULL

This information indicates that thread 10 is the I/O thread that is communicating with the master server, and thread 11 is the SQL thread that is processing the updates stored in the relay logs. Currently, both threads are idle, waiting for further updates.

Note that the value in the Time column can tell how late the slave is compared to the master. See section 6.9 Replication FAQ.

6.3.1 Replication Master Thread States

The following list shows the most common states you will see in the State column for the master's Binlog Dump thread. If you don't see any Binlog Dump threads on a master server, replication is not running. That is, no slaves currently are connected.

Sending binlog event to slave
Binary logs consist of events, where an event is usually an update statement plus some other information. The thread has read an event from the binary log and is sending it to the slave.
Finished reading one binlog; switching to next binlog
The thread has finished reading a binary log file and is opening the next one to send to the slave.
Has sent all binlog to slave; waiting for binlog to be updated
The thread has read all outstanding updates from the binary logs and sent them to the slave. It is idle, waiting for new events to appear in the binary log resulting from new update statements being executed on the master.
Waiting to finalize termination
A very brief state that occurs as the thread is stopping.

6.3.2 Replication Slave I/O Thread States

The following list shows the most common states you will see in the State column for a slave server I/O thread. Beginning with MySQL 4.1.1, this state also appears in the Slave_IO_State column displayed by the SHOW SLAVE STATUS statement. This means that you can get a good view of what is happening by using only SHOW SLAVE STATUS.

Connecting to master
The thread is attempting to connect to the master.
Checking master version
A very brief state that occurs just after the connection to the master is established.
Registering slave on master
A very brief state that occurs just after the connection to the master is established.
Requesting binlog dump
A very brief state that occurs just after the connection to the master is established. The thread sends to the master a request for the contents of its binary logs, starting from the requested binary log filename and position.
Waiting to reconnect after a failed binlog dump request
If the binary log dump request failed (due to disconnection), the thread goes into this state while it sleeps, then tries to reconnect periodically. The interval between retries can be specified using the --master-connect-retry option.
Reconnecting after a failed binlog dump request
The thread is trying to reconnect to the master.
Waiting for master to send event
The thread has connected to the master and is waiting for binary log events to arrive. This can last for a long time if the master is idle. If the wait lasts for slave_read_timeout seconds, a timeout will occur. At that point, the thread will consider the connection to be broken and make an attempt to reconnect.
Queueing master event to the relay log
The thread has read an event and is copying it to the relay log so that the SQL thread can process it.
Waiting to reconnect after a failed master event read
An error occurred while reading (due to disconnection). The thread is sleeping for master-connect-retry seconds before attempting to reconnect.
Reconnecting after a failed master event read
The thread is trying to reconnect to the master. When connection is established again, the state will become Waiting for master to send event.
Waiting for the slave SQL thread to free enough relay log space
You are using a non-zero relay_log_space_limit value, and the relay logs have grown so much that their combined size exceeds this value. The I/O thread is waiting until the SQL thread frees enough space by processing relay log contents so that it can delete some relay log files.
Waiting for slave mutex on exit
A very brief state that occurs as the thread is stopping.

6.3.3 Replication Slave SQL Thread States

The following list shows the most common states you will see in the State column for a slave server SQL thread:

Reading event from the relay log
The thread has read an event from the relay log so that it can process it.
Has read all relay log; waiting for the slave I/O thread to update it
The thread has processed all events in the relay log files and is waiting for the I/O thread to write new events to the relay log.
Waiting for slave mutex on exit
A very brief state that occurs as the thread is stopping.

The State column for the I/O thread may also show the text of a statement. This indicates that the thread has read an event from the relay log, extracted the statement from it, and is executing it.

6.3.4 Replication Relay and Status Files

By default, relay logs are named using filenames of the form `host_name-relay-bin.nnnnnn', where host_name is the name of the slave server host and nnnnnn is a sequence number. Successive relay log files are created using successive sequence numbers, beginning with 000001 (001 in MySQL 4.0 or older). The slave keeps track of relay logs currently in use in an index file. The default relay log index filename is `host_name-relay-bin.index'. By default, these files are created in the slave's data directory. The default filenames may be overridden with the --relay-log and --relay-log-index server options. See section 6.8 Replication Startup Options.

Relay logs have the same format as binary logs, so you can use mysqlbinlog to read them. A relay log is automatically deleted by the SQL thread as soon as it has executed all its events and no longer needs it). There is no explicit mechanism for deleting relay logs, because the SQL thread takes care of doing so. However, from MySQL 4.0.14, FLUSH LOGS rotates relay logs, which will influence when the SQL thread deletes them.

A new relay log is created under the following conditions:

A slave replication server creates two additional small files in the data directory. These are status files and are named `master.info' and `relay-log.info' by default. They contain information like that shown in the output of the SHOW SLAVE STATUS statement (see section 13.6.2 SQL Statements for Controlling Slave Servers for a description of this statement). As disk files, they survive a slave server's shutdown. The next time the slave starts up, it reads these files to determine how far it has proceeded in reading binary logs from the master and in processing its own relay logs.

The `master.info' file is updated by the I/O thread. Before MySQL 4.1, the correspondence between the lines in the file and the columns displayed by SHOW SLAVE STATUS is as follows:

Line Description
1 Master_Log_File
2 Read_Master_Log_Pos
3 Master_Host
4 Master_User
5 Password (not shown by SHOW SLAVE STATUS)
6 Master_Port
7 Connect_Retry

As of MySQL 4.1, the file includes a line count and information about SSL options:

Line Description
1 Number of lines in the file
2 Master_Log_File
3 Read_Master_Log_Pos
4 Master_Host
5 Master_User
6 Password (not shown by SHOW SLAVE STATUS)
7 Master_Port
8 Connect_Retry
9 Master_SSL_Allowed
10 Master_SSL_CA_File
11 Master_SSL_CA_Path
12 Master_SSL_Cert
13 Master_SSL_Cipher
14 Master_SSL_Key

The `relay-log.info' file is updated by the SQL thread. The correspondence between the lines in the file and the columns displayed by SHOW SLAVE STATUS is as follows:

Line Description
1 Relay_Log_File
2 Relay_Log_Pos
3 Relay_Master_Log_File
4 Exec_Master_Log_Pos

When you back up your slave's data, you should back up these two small files as well, along with the relay log files. They are needed to resume replication after you restore the slave's data. If you lose the relay logs but still have the `relay-log.info' file, you can check it to determine how far the SQL thread has executed in the master binary logs. Then you can use CHANGE MASTER TO with the MASTER_LOG_FILE and MASTER_LOG_POS options to tell the slave to re-read the binary logs from that point. This requires that the binary logs still exist on the master server.

If your slave is subject to replicating LOAD DATA INFILE statements, you should also back up any `SQL_LOAD-*' files that exist in the directory that the slave uses for this purpose. The slave needs these files to resume replication of any interrupted LOAD DATA INFILE operations. The directory location is specified using the --slave-load-tmpdir option. Its default value, if not specified, is the value of the tmpdir variable.

6.4 How to Set Up Replication

Here is a quick description of how to set up complete replication of your current MySQL server. It assumes that you want to replicate all your databases and have not configured replication before. You will need to shut down your master server briefly to complete the steps outlined here.

The procedure is written in terms of setting up a single slave, but you can use it to set up multiple slaves.

While this method is the most straightforward way to set up a slave, it is not the only one. For example, if you already have a snapshot of the master's data, and the master already has its server ID set and binary logging enabled, you can set up a slave without shutting down the master or even blocking updates to it. For more details, please see section 6.9 Replication FAQ.

If you want to administer a MySQL replication setup, we suggest that you read this entire chapter through and try all statements mentioned in section 13.6.1 SQL Statements for Controlling Master Servers and section 13.6.2 SQL Statements for Controlling Slave Servers. You should also familiarize yourself with replication startup options described in section 6.8 Replication Startup Options.

Note that this procedure and some of the replication SQL statements in later sections refer to the SUPER privilege. Prior to MySQL 4.0.2, use the PROCESS privilege instead.

  1. Make sure that you have a recent version of MySQL installed on the master and slaves, and that these versions are compatible according to the table shown in section 6.5 Replication Compatibility Between MySQL Versions. Please do not report bugs until you have verified that the problem is present in the latest release.
  2. Set up an account on the master server that the slave server can use to connect. This account must be given the REPLICATION SLAVE privilege. If the account is used only for replication (which is recommended), you don't need to grant any additional privileges. Suppose that your domain is mydomain.com and you want to create an account with a username of repl such that slave servers can use the account to access the master server from any host in your domain using a password of slavepass. To create the account, this use GRANT statement:
    mysql> GRANT REPLICATION SLAVE ON *.*
        -> TO 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
    
    For MySQL versions older than 4.0.2, the REPLICATION SLAVE privilege does not exist. Grant the FILE privilege instead:
    mysql> GRANT FILE ON *.*
        -> TO 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
    
    If you plan to use the LOAD TABLE FROM MASTER or LOAD DATA FROM MASTER statements from the slave host, you will need to grant this account additional privileges:
  3. If you are using only MyISAM tables, flush all the tables and block write statements by executing a FLUSH TABLES WITH READ LOCK statement.
    mysql> FLUSH TABLES WITH READ LOCK;
    
    Leave the client running from which you issue the FLUSH TABLES statement so that the read lock remains in effect. (If you exit the client, the lock is released.) Then take a snapshot of the data on your master server. The easiest way to create a snapshot is to use an archiving program to make a binary backup of the databases in your master's data directory. For example, use tar on Unix, or PowerArchiver, WinRAR, WinZip, or any similar software on Windows. To use tar to create an archive that includes all databases, change location into the master server's data directory, then execute this command:
    shell> tar -cvf /tmp/mysql-snapshot.tar .
    
    If you want the archive to include only a database called this_db, use this command instead:
    shell> tar -cvf /tmp/mysql-snapshot.tar ./this_db
    
    Then copy the archive file to the `/tmp' directory on the slave server host. On that machine, change location into the slave's data directory, and unpack the archive file using this command:
    shell> tar -xvf /tmp/mysql-snapshot.tar
    
    You may not want to replicate the mysql database if the slave server has a different set of user accounts from those that exist on the master. In this case, you should exclude it from the archive. You also need not include any log files in the archive, or the `master.info' or `relay-log.info' files. While the read lock placed by FLUSH TABLES WITH READ LOCK is in effect, read the value of the current binary log name and offset on the master:
    mysql > SHOW MASTER STATUS;
    +---------------+----------+--------------+------------------+
    | File          | Position | Binlog_Do_DB | Binlog_Ignore_DB |
    +---------------+----------+--------------+------------------+
    | mysql-bin.003 | 73       | test         | manual,mysql     |
    +---------------+----------+--------------+------------------+
    
    The File column shows the name of the log, while Position shows the offset. In this example, the binary log value is mysql-bin.003 and the offset is 73. Record the values. You will need to use them later when you are setting up the slave. They represent the replication coordinates at which the slave should begin processing new updates from the master. After you have taken the snapshot and recorded the log name and offset, you can re-enable write activity on the master:
    mysql> UNLOCK TABLES;
    
    If you are using InnoDB tables, ideally you should use the InnoDB Hot Backup tool. It takes a consistent snapshot without acquiring any locks on the master server, and records the log name and offset corresponding to the snapshot to be later used on the slave. InnoDB Hot Backup is a non-free (commercial) additional tool that is not included in the standard MySQL distribution. See the InnoDB Hot Backup home page at http://www.innodb.com/manual.php for detailed information and screenshots. Without the Hot Backup tool, the quickest way to take a binary snapshot of InnoDB tables is to shut down the master server and copy the InnoDB data files, log files, and table definition files (`.frm' files). To record the current log file name and offset, you should issue the following statements before you shut down the server:
    mysql> FLUSH TABLES WITH READ LOCK;
    mysql> SHOW MASTER STATUS;
    
    Then record the log name and the offset from the output of SHOW MASTER STATUS as was shown earlier. After recording the log name and the offset, shut down the server without unlocking the tables to make sure that the server goes down with the snapshot corresponding to the current log file and offset:
    shell> mysqladmin -u root shutdown
    
    An alternative that works for both MyISAM and InnoDB tables is to take an SQL dump of the master instead of a binary copy as described in the preceding discussion. For this, you can use mysqldump --master-data on your master and later load the SQL dump file into your slave. However, this is slower than doing a binary copy. If the master has been previously running without --log-bin enabled, the log name and position values displayed by SHOW MASTER STATUS or mysqldump --master-data will be empty. In that case, the values that you will need to use later when specifying the slave's log file and position are the empty string ('') and 4.
  4. Make sure that the [mysqld] section of the `my.cnf' file on the master host includes a log-bin option. The section should also have a server-id=master_id option, where master_id must be a positive integer value from 1 to 2^32 - 1. For example:
    [mysqld]
    log-bin
    server-id=1
    
    If those options are not present, add them and restart the server.
  5. Stop the server that is to be used as a slave server and add the following to its `my.cnf' file:
    [mysqld]
    server-id=slave_id
    
    The slave_id value, like the master_id value, must be a positive integer value from 1 to 2^32 - 1. In addition, it is very important that the ID of the slave be different from the ID of the master. For example:
    [mysqld]
    server-id=2
    
    If you are setting up multiple slaves, each one must have a unique server-id value that differs from that of the master and from each of the other slaves. Think of server-id values as something similar to IP addresses: These IDs uniquely identify each server instance in the community of replication partners. If you don't specify a server-id value, it will be set to 1 if you have not defined master-host, else it will be set to 2. Note that in the case of server-id omission, a master will refuse connections from all slaves, and a slave will refuse to connect to a master. Thus, omitting server-id is good only for backup with a binary log.
  6. If you made a binary backup of the master server's data, copy it to the slave server's data directory before starting the slave. Make sure that the privileges on the files and directories are correct. The user that the server MySQL runs as must able to read and write the files, just as on the master. If you made a backup using mysqldump, start the slave first (see next step).
  7. Start the slave server. If it has been replicating previously, start the slave server with the --skip-slave-start option so that it doesn't immediately try to connect to its master. You also may want to start the slave server with the --log-warnings option (enabled by default as of MySQL 4.0.19 and 4.1.2), to get more messages in the error log about problems (for example, network or connection problems). As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1.
  8. If you made a backup of the master server's data using mysqldump, load the dump file into the slave server:
    shell> mysql -u root -p < dump_file.sql
    
  9. Execute the following statement on the slave, replacing the option values with the actual values relevant to your system:
    mysql> CHANGE MASTER TO
        ->     MASTER_HOST='master_host_name',
        ->     MASTER_USER='replication_user_name',
        ->     MASTER_PASSWORD='replication_password',
        ->     MASTER_LOG_FILE='recorded_log_file_name',
        ->     MASTER_LOG_POS=recorded_log_position;
    
    The following table shows the maximum length for the string options:
    MASTER_HOST 60
    MASTER_USER 16
    MASTER_PASSWORD 32
    MASTER_LOG_FILE 255
  10. Start the slave threads:
    mysql> START SLAVE; 
    

After you have performed this procedure, the slave should connect to the master and catch up on any updates that have occurred since the snapshot was taken.

If you have forgotten to set the server-id value for the master, slaves will not be able to connect to it.

If you have forgotten to set the server-id value for the slave, you will get the following error in its error log:

Warning: You should set server-id to a non-0 value if master_host is set;
we force server id to 2, but this MySQL server will not act as a slave.

You will also find error messages in the slave's error log if it is not able to replicate for any other reason.

Once a slave is replicating, you will find in its data directory one file named `master.info' and another named `relay-log.info'. The slave uses these two files to keep track of how much of the master's binary log it has processed. Do not remove or edit these files, unless you really know what you are doing and understand the implications. Even in that case, it is preferred that you use the CHANGE MASTER TO statement.

Note: The content of `master.info' overrides some options specified on the command line or in `my.cnf'. See section 6.8 Replication Startup Options for more details.

Once you have a snapshot, you can use it to set up other slaves by following the slave portion of the procedure just described. You do not need to take another snapshot of the master; you can use the same one for each slave.

6.5 Replication Compatibility Between MySQL Versions

The original binary log format was developed in MySQL 3.23. It changed in MySQL 4.0, and again in MySQL 5.0. This has consequences when you upgrade servers in a replication setup, as described in section 6.6 Upgrading a Replication Setup.

As far as replication is concerned, any MySQL 4.1.x version and any 4.0.x version are identical, because they all use the same binary log format. Thus, any servers from these versions are compatible, and replication between them should work seamlessly. The exceptions to this compatibility is that versions from MySQL 4.0.0 to 4.0.2 were very early development versions that should not be used anymore. (These were the alpha versions in the 4.0 release series. Compatibility for them is still documented in the manual included with their distributions.)

The following table indicates master/slave replication compatibility between different versions of MySQL.

Master Master Master
3.23.33 and up 4.0.3 and up or any 4.1.x 5.0.0
Slave 3.23.33 and up yes no no
Slave 4.0.3 and up yes yes no
Slave 5.0.0 yes yes yes

As a general rule, we recommended using recent MySQL versions, because replication capabilities are continually being improved. We also recommend using the same version for both the master and the slave.

6.6 Upgrading a Replication Setup

When you upgrade servers that participate in a replication setup, the procedure for upgrading depends on the current server versions and the version to which you are upgrading.

6.6.1 Upgrading Replication to 4.0 or 4.1

This section applies to upgrading replication from MySQL 3.23 to 4.0 or 4.1. A 4.0 server should be 4.0.3 or newer, as mentioned in section 6.5 Replication Compatibility Between MySQL Versions.

When you upgrade a master from MySQL 3.23 to MySQL 4.0 or 4.1, you should first ensure that all the slaves of this master are already at 4.0 or 4.1. If that is not the case, you should first upgrade your slaves: Shut down each one, upgrade it, restart it, and restart replication.

The upgrade can safely be done using the following procedure, assuming that you have a 3.23 master to upgrade and the slaves are 4.0 or 4.1. Note that after the master has been upgraded, you should not restart replication using any old 3.23 binary logs, because this will unfortunately confuse the 4.0 or 4.1 slave.

  1. Block all updates on the master by issuing a FLUSH TABLES WITH READ LOCK statement.
  2. Wait until all the slaves have caught up with all changes from the master server. Use SHOW MASTER STATUS on the master to obtain its current binary log file and position. Then, for each slave, use those values with a SELECT MASTER_POS_WAIT() statement. The statement will block on the slave and return when the slave has caught up. Then run STOP SLAVE on the slave.
  3. Stop the master server and upgrade it to MySQL 4.0 or 4.1.
  4. Restart the master server and record the name of its newly created binary log. You can obtain the name of the file by issuing a SHOW MASTER STATUS statement on the master. Then issue these statements on each slave:
    mysql> CHANGE MASTER TO MASTER_LOG_FILE='binary_log_name',
        ->     MASTER_LOG_POS=4;
    mysql> START SLAVE;
    

6.6.2 Upgrading Replication to 5.0

This section applies to upgrading replication from MySQL 3.23, 4.0, or 4.1 to 5.0.0. A 4.0 server should be 4.0.3 or newer, as mentioned in section 6.5 Replication Compatibility Between MySQL Versions.

First, note that MySQL 5.0.0 is an alpha release. It is intended to work better than older versions (easier upgrade, replication of some important session variables such as sql_mode; see section D.1.4 Changes in release 5.0.0 (22 Dec 2003: Alpha)). However it has not yet been extensively tested. As with any alpha release, we recommend that you not use it in critical production environments yet.

When you upgrade a master from MySQL 3.23, 4.0, or 4.1 to 5.0.0, you should first ensure that all the slaves of this master are already 5.0.0. If that's not the case, you should first upgrade your slaves. To upgrade each slave, just shut it down, upgrade it to 5.0.0, restart it, and restart replication. The 5.0.0 slave will be able to read its old relay logs that were written before the upgrade and execute the statements they contain. Relay logs created by the slave after the upgrade will be in 5.0.0 format.

After the slaves have been upgraded, shut down your master, upgrade it to 5.0.0, and restart it. The 5.0.0 master will be able to read its old binary logs that were written before the upgrade and send them to the 5.0.0 slaves. The slaves will recognize the old format and handle it properly. Binary logs created by master after the upgrade will be in 5.0.0 format. These too will be recognized by the 5.0.0 slaves.

In other words, there are no measures to take when upgrading to 5.0.0, except that slaves must be 5.0.0 before you can upgrade the master to 5.0.0. Note that downgrading from 5.0.0 to older versions does not work so automatically: You must ensure that any 5.0.0 binary logs or relay logs have been fully processed, so that you can remove them before proceeding with the downgrade.

6.7 Replication Features and Known Problems

The following list explains what is supported and what is not. Additional InnoDB-specific information about replication is given in section 15.7.5 InnoDB and MySQL Replication.

The following table lists replication problems in MySQL 3.23 that are fixed in MySQL 4.0:

6.8 Replication Startup Options

On both the master and the slave, you must use the server-id option to establish a unique replication ID for each server. You should pick a unique positive integer in the range from 1 to 2^32 - 1 for each master and slave. Example: server-id=3

The options that you can use on the master server for controlling binary logging are described in section 5.9.4 The Binary Log.

The following table describes the options you can use on slave replication servers. You can specify them on the command line or in an option file.

Some slave server replication options are handled in a special way, in the sense that they are ignored if a `master.info' file exists when the slave starts and contains values for the options. The following options are handled this way:

As of MySQL 4.1.1, the following options also are handled specially:

The `master.info' file format in 4.1.1 changed to include values corresponding to the SSL options. In addition, the 4.1.1 file format includes as its first line the number of lines in the file. If you upgrade an older server to 4.1.1, the new server upgrades the `master.info' file to the new format automatically when it starts. However, if you downgrade a 4.1.1 or newer server to a version older than 4.1.1, you should manually remove the first line before starting the older server for the first time. Note that, in this case, the downgraded server no longer can use an SSL connection to communicate with the master.

If no `master.info' file exists when the slave server starts, it uses values for those options that are specified in option files or on the command line. This will occur when you start the server as a replication slave for the very first time, or when you have run RESET SLAVE and shut down and restarted the slave server.

If the `master.info' file exists when the slave server starts, the server ignores those options. Instead, it uses the values found in the `master.info' file.

If you restart the slave server with different values of the startup options that correspond to values in the `master.info' file, the different values have no effect, because the server continues to use the `master.info' file. To use different values, you must either restart after removing the `master.info' file or (preferably) use the CHANGE MASTER TO statement to reset the values while the slave is running.

Suppose that you specify this option in your `my.cnf' file:

[mysqld]
master-host=some_host

The first time you start the server as a replication slave, it reads and uses that option from the `my.cnf' file. The server then records the value in the `master.info' file. The next time you start the server, it reads the master host value from the `master.info' file only and ignores the value in the option file. If you modify the `my.cnf' file to specify a different master host of some_other_host, the change still will have no effect. You should use CHANGE MASTER TO instead.

Because the server gives an existing `master.info' file precedence over the startup options just described, you might prefer not to use startup options for these values at all, and instead specify them by using the CHANGE MASTER TO statement. See section 13.6.2.1 CHANGE MASTER TO Syntax.

This example shows a more extensive use of startup options to configure a slave server:

[mysqld]
server-id=2
master-host=db-master.mycompany.com
master-port=3306
master-user=pertinax
master-password=freitag
master-connect-retry=60
report-host=db-slave.mycompany.com

The following list describes startup options for controlling replication: Many of these options can be reset while the server is running by using the CHANGE MASTER TO statement. Others, such as the --replicate-* options, can be set only when the slave server starts. We plan to fix this.

--log-slave-updates
Normally, updates received from a master server by a slave are not logged to its binary log. This option tells the slave to log the updates performed by its SQL thread to the slave's own binary log. For this option to have any effect, the slave must also be started with the --log-bin option to enable binary logging. --log-slave-updates is used when you want to chain replication servers. For example, you might want a setup like this:
A -> B -> C
That is, A serves as the master for the slave B, and B serves as the master for the slave C. For this to work, B must be both a master and a slave. You must start both A and B with --log-bin to enable binary logging, and B with the --log-slave-updates option.
--log-warnings
Makes the slave print more messages to the error log about what it is doing. For example, it will warn you that it succeeded in reconnecting after a network/connection failure, and warn you about how each slave thread started. This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --skip-log-warnings. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1. This option is not limited to replication use only. It produces warnings across a spectrum of server activities.
--master-connect-retry=seconds
The number of seconds the slave thread sleeps before retrying to connect to the master in case the master goes down or the connection is lost. The value in the `master.info' file takes precedence if it can be read. If not set, the default is 60.
--master-host=host
The hostname or IP number of the master replication server. If this option is not given, the slave thread will not be started. The value in `master.info' takes precedence if it can be read.
--master-info-file=file_name
The name to use for the file in which the slave records information about the master. The default name is `mysql.info' in the data directory.
--master-password=password
The password of the account that the slave thread uses for authentication when connecting to the master. The value in the `master.info' file takes precedence if it can be read. If not set, an empty password is assumed.
--master-port=port_number
The TCP/IP port the master is listening on. The value in the `master.info' file takes precedence if it can be read. If not set, the compiled-in setting is assumed. If you have not tinkered with configure options, this should be 3306.
--master-ssl
--master-ssl-ca=file_name
--master-ssl-capath=directory_name
--master-ssl-cert=file_name
--master-ssl-cipher=cipher_list
--master-ssl-key=file_name
These options are used for setting up a secure replication connection to the master server using SSL. Their meanings are the same as the corresponding --ssl, --ssl-ca, --ssl-capath, --ssl-cert, --ssl-cipher, --ssl-key options described in section 5.6.7.5 SSL Command-Line Options. The values in the `master.info' file take precedence if they can be read. These options are operational as of MySQL 4.1.1.
--master-user=username
The username of the account that the slave thread uses for authentication when connecting to the master. The account must have the REPLICATION SLAVE privilege. (Prior to MySQL 4.0.2, it must have the FILE privilege instead.) The value in the `master.info' file takes precedence if it can be read. If the master user is not set, user test is assumed.
--max-relay-log-size=#
To rotate the relay log automatically. See section 5.2.3 Server System Variables. This option is available as of MySQL 4.0.14.
--read-only
This option causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This option is available as of MySQL 4.0.14.
--relay-log=file_name
The name for the relay log. The default name is host_name-relay-bin.nnnnnn, where host_name is the name of the slave server host and nnnnnn indicates that relay logs are created in numbered sequence. You can specify the option to create hostname-independent relay log names, or if your relay logs tend to be big (and you don't want to decrease max_relay_log_size) and you need to put them in some area different from the data directory, or if you want to increase speed by balancing load between disks.
--relay-log-index=file_name
The location and name that should be used for the relay log index file. The default name is host_name-relay-bin.index, where host_name is the name of the slave server.
--relay-log-info-file=file_name
The name to use for the file in which the slave records information about the relay logs. The default name is `relay-log.info' in the data directory.
--relay-log-purge={0|1}
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (enabled). This is a global variable that can be changed dynamically with SET GLOBAL relay_log_purge. This option is available as of MySQL 4.1.1.
--relay-log-space-limit=#
Places an upper limit on the total size of all relay logs on the slave (a value of 0 means ``unlimited''). This is useful for a slave server host that has limited disk space. When the limit is reached, the I/O thread stops reading binary log events from the master server until the SQL thread has caught up and deleted some now unused relay logs. Note that this limit is not absolute: There are cases where the SQL thread needs more events before it can delete relay logs. In that case, the I/O thread will exceed the limit until it becomes possible for the SQL thread to delete some relay logs. Not doing so would cause a deadlock (which is what happens before MySQL 4.0.13). You should not set --relay-log-space-limit to less than twice the value of --max-relay-log-size (or --max-binlog-size if --max-relay-log-size is 0). In that case, there is a chance that the I/O thread will wait for free space because --relay-log-space-limit is exceeded, but the SQL thread will have no relay log to purge and be unable to satisfy the I/O thread. This forces the I/O thread to temporarily ignore --relay-log-space-limit.
--replicate-do-db=db_name
Tells the slave to restrict replication to statements where the default database (that is, the one selected by USE) is db_name. To specify more than one database, use this option multiple times, once for each database. Note that this will not replicate cross-database statements such as UPDATE some_db.some_table SET foo='bar' while having selected a different database or no database. If you need cross-database updates to work, make sure that you have MySQL 3.23.28 or later, and use --replicate-wild-do-table=db_name.%. Please read the notes that follow this option list. An example of what does not work as you might expect: If the slave is started with --replicate-do-db=sales and you issue the following statements on the master, the UPDATE statement will not be replicated:
USE prices;
UPDATE sales.january SET amount=amount+1000;
If you need cross-database updates to work, use --replicate-wild-do-table=db_name.% instead. The main reason for this ``just-check-the-default-database'' behavior is that it's difficult from the statement alone to know whether or not it should be replicated (for example, if you are using multiple-table DELETE or multiple-table UPDATE statements that go across multiple databases). It's also very fast to just check the default database.
--replicate-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to the specified table. To specify more than one table, use this option multiple times, once for each table. This will work for cross-database updates, in contrast to --replicate-do-db. Please read the notes that follow this option list.
--replicate-ignore-db=db_name
Tells the slave to not replicate any statement where the default database (that is, the one selected by USE) is db_name. To specify more than one database to ignore, use this option multiple times, once for each database. You should not use this option if you are using cross-database updates and you don't want these updates to be replicated. Please read the notes that follow this option list. An example of what does not work as you might expect: If the slave is started with --replicate-ignore-db=sales and you issue the following statements on the master, the UPDATE statement will be replicated:
USE prices;
UPDATE sales.january SET amount=amount+1000;
If you need cross-database updates to work, use --replicate-wild-ignore-table=db_name.% instead.
--replicate-ignore-table=db_name.tbl_name
Tells the slave thread to not replicate any statement that updates the specified table (even if any other tables might be updated by the same statement). To specify more than one table to ignore, use this option multiple times, once for each table. This will work for cross-database updates, in contrast to --replicate-ignore-db. Please read the notes that follow this option list.
--replicate-wild-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to statements where any of the updated tables match the specified database and table name patterns. Patterns can contain the `%' and `_' wildcard characters, which have the same meaning as for the LIKE pattern-matching operator. To specify more than one table, use this option multiple times, once for each table. This will work for cross-database updates. Please read the notes that follow this option list. Example: --replicate-wild-do-table=foo%.bar% will replicate only updates that use a table where the database name starts with foo and the table name starts with bar. If the table name pattern is %, it matches any table name and the option also applies to database-level statements (CREATE DATABASE, DROP DATABASE, and ALTER DATABASE). For example, if you use --replicate-wild-do-table=foo%.%, database-level statements are replicated if the database name matches the pattern foo%. To include literal wildcard characters in the database or table name patterns, escape them with a backslash. For example, to replicate all tables of a database that is named my_own%db, but not replicate tables from the my1ownAABCdb database, you should escape the `_' and `%' characters like this: --replicate-wild-do-table=my\_own\%db. If you're using the option on the command line, you might need to double the backslashes or quote the option value, depending on your command interpreter. For example, with the bash shell, you would need to type --replicate-wild-do-table=my\\_own\\%db.
--replicate-wild-ignore-table=db_name.tbl_name
Tells the slave thread to not replicate a statement where any table matches the given wildcard pattern. To specify more than one table to ignore, use this option multiple times, once for each table. This will work for cross-database updates. Please read the notes that follow this option list. Example: --replicate-wild-ignore-table=foo%.bar% will not replicate updates that use a table where the database name starts with foo and the table name starts with bar. For information about how matching works, see the description of the --replicate-wild-ignore-table option. The rules for including literal wildcard characters in the option value are the same as for --replicate-wild-ignore-table as well.
--replicate-rewrite-db=from_name->to_name
Tells the slave to translate the default database (that is, the one selected by USE) to to_name if it was from_name on the master. Only statements involving tables are affected (not statements such as CREATE DATABASE, DROP DATABASE, and ALTER DATABASE), and only if from_name was the default database on the master. This will not work for cross-database updates. Note that the database name translation is done before --replicate-* rules are tested. If you use this option on the command line and the `>' character is special to your command interpreter, quote the option value. For example:
shell> mysqld --replicate-rewrite-db="olddb->newdb"
--replicate-same-server-id
To be used on slave servers. Usually you can should the default setting of 0, to prevent infinite loops in circular replication. If set to 1, this slave will not skip events having its own server id; normally this is useful only in rare configurations. Cannot be set to 1 if --log-slave-updates is used. Be careful that starting from MySQL 4.1, by default the slave I/O thread does not even write binary log events to the relay log if they have the slave's server id (this optimization helps save disk usage compared to 4.0). So if you want to use --replicate-same-server-id in 4.1 versions, be sure to start the slave with this option before you make the slave read its own events which you want the slave SQL thread to execute.
--report-host=host
The hostname or IP number of the slave to be reported to the master during slave registration. This value will appear in the output of SHOW SLAVE HOSTS on the master server. Leave the value unset if you do not want the slave to register itself with the master. Note that it is not sufficient for the master to simply read the IP number of the slave from the TCP/IP socket after the slave connects. Due to NAT and other routing issues, that IP may not be valid for connecting to the slave from the master or other hosts. This option is available as of MySQL 4.0.0.
--report-port=port_number
The TCP/IP port for connecting to the slave, to be reported to the master during slave registration. Set it only if the slave is listening on a non-default port or if you have a special tunnel from the master or other clients to the slave. If you are not sure, leave this option unset. This option is available as of MySQL 4.0.0.
--skip-slave-start
Tells the slave server not to start the slave threads when the server starts. To start the threads later, use a START SLAVE statement.
--slave_compressed_protocol={0|1}
If this option is set to 1, use compression of the slave/master protocol if both the slave and the master support it.
--slave-load-tmpdir=file_name
The name of the directory where the slave creates temporary files. This option is by default equal to the value of the tmpdir system variable. When the slave SQL thread replicates a LOAD DATA INFILE statement, it extracts the to-be-loaded file from the relay log into temporary files, then loads these into the table. If the file loaded on the master was huge, the temporary files on the slave will be huge, too. Therefore, it might be advisable to use this option to tell the slave to put temporary files in a directory located in some filesystem that has a lot of available space. In that case, you may also use the --relay-log option to place the relay logs in that filesystem, because the relay logs will be huge as well. --slave-load-tmpdir should point to a disk-based filesystem, not a memory-based one: The slave needs the temporary files used to replicate LOAD DATA INFILE to survive a machine's restart. The directory also should not be one that is cleared by the operating system during the system startup process.
--slave-net-timeout=seconds
The number of seconds to wait for more data from the master before aborting the read, considering the connection broken, and trying to reconnect. The first retry occurs immediately after the timeout. The interval between retries is controlled by the --master-connect-retry option.
--slave-skip-errors= [err_code1,err_code2,... | all]
Normally, replication stops when an error occurs, which gives you the opportunity to resolve the inconsistency in the data manually. This option tells the slave SQL thread to continue replication when a statement returns any of the errors listed in the option value. Do not use this option unless you fully understand why you are getting the errors. If there are no bugs in your replication setup and client programs, and no bugs in MySQL itself, an error that stops replication should never occur. Indiscriminate use of this option will result in slaves becoming hopelessly out of sync with the master, and you will have no idea why. For error codes, you should use the numbers provided by the error message in your slave error log and in the output of SHOW SLAVE STATUS. The server error codes are listed in section 23 Error Handling in MySQL. You can (but should not) also use the very non-recommended value of all which will ignore all error messages and keep barging along regardless of what happens. Needless to say, if you use it, we make no promises regarding your data integrity. Please do not complain if your data on the slave is not anywhere close to what it is on the master in this case. You have been warned. Examples:
--slave-skip-errors=1062,1053
--slave-skip-errors=all

The --replicate-* rules are evaluated as follows to determine whether a statement will be executed by the slave or ignored:

  1. Are there some --replicate-do-db or --replicate-ignore-db rules?
  2. Are there some --replicate-*-table rules?
  3. Are there some --replicate-do-table rules?
  4. Are there some --replicate-ignore-table rules?
  5. Are there some --replicate-wild-do-table rules?
  6. Are there some --replicate-wild-ignore-table rules?
  7. No --replicate-*-table rule was matched. Is there another table to test against these rules?

6.9 Replication FAQ

Q: How do I configure a slave if the master is already running and I do not want to stop it?

A: There are several options. If you have taken a backup of the master at some point and recorded the binary log name and offset (from the output of SHOW MASTER STATUS ) corresponding to the snapshot, use the following procedure:

  1. Make sure that the slave is assigned a unique server ID.
  2. Execute the following statement on the slave, filling in appropriate values for each option:
    mysql> CHANGE MASTER TO
        ->     MASTER_HOST='master_host_name',
        ->     MASTER_USER='master_user_name',
        ->     MASTER_PASSWORD='master_pass',
        ->     MASTER_LOG_FILE='recorded_log_file_name',
        ->     MASTER_LOG_POS=recorded_log_position;
    
  3. Execute START SLAVE on the slave.

If you do not have a backup of the master server already, here is a quick procedure for creating one. All steps should be performed on the master host.

  1. Issue this statement:
    mysql> FLUSH TABLES WITH READ LOCK;
    
  2. With the lock still in place, execute this command (or a variation of it):
    shell> tar zcf /tmp/backup.tar.gz /var/lib/mysql
    
  3. Issue this statement and make sure to record the output, which you will need later:
    mysql> SHOW MASTER STATUS;
    
  4. Release the lock:
    mysql> UNLOCK TABLES;
    

An alternative is to make an SQL dump of the master instead of a binary copy as in the preceding procedure. To do this, you can use mysqldump --master-data on your master and later load the SQL dump into your slave. However, this is slower than making a binary copy.

No matter which of the two methods you use, afterward follow the instructions for the case when you have a snapshot and have recorded the log name and offset. You can use the same snapshot to set up several slaves. Once you have the snapshot of the master, you can wait to set up a slave as long as the binary logs of the master are left intact. The two practical limitations on the length of time you can wait are the amount of disk space available to retain binary logs on the master and the length of time it will take the slave to catch up.

You can also use LOAD DATA FROM MASTER. This is a convenient statement that transfers a snapshot to the slave and adjusts the log name and offset all at once. In the future, LOAD DATA FROM MASTER will be the recommended way to set up a slave. Be warned, however, that it works only for MyISAM tables and it may hold a read lock for a long time. It is not yet implemented as efficiently as we would like. If you have large tables, the preferred method at this time is still to make a binary snapshot on the master server after executing FLUSH TABLES WITH READ LOCK.

Q: Does the slave need to be connected to the master all the time?

A: No, it does not. The slave can go down or stay disconnected for hours or even days, then reconnect and catch up on the updates. For example, you can set up a master/slave relationship over a dial-up link where the link is up only sporadically and for short periods of time. The implication of this is that, at any given time, the slave is not guaranteed to be in sync with the master unless you take some special measures. In the future, we will have the option to block the master until at least one slave is in sync.

Q: How do I know how late a slave is compared to the master? In other words, how do I know the date of the last query replicated by the slave?

A: If the slave is 4.1.1 or newer, read the Seconds_Behind_Master column in SHOW SLAVE STATUS. For older versions, the following applies. This is possible only if SHOW SLAVE STATUS on the slave shows that the SQL thread is running (or for MySQL 3.23, that the slave thread is running), and that the thread has executed at least one event from the master. See section 6.3 Replication Implementation Details.

When the slave SQL thread executes an event read from the master, it modifies its own time to the event timestamp (this is why TIMESTAMP is well replicated). In the Time column in the output of SHOW PROCESSLIST, the number of seconds displayed for the slave SQL thread is the number of seconds between the timestamp of the last replicated event and the real time of the slave machine. You can use this to determine the date of the last replicated event. Note that if your slave has been disconnected from the master for one hour, and then reconnects, you may immediately see Time values like 3600 for the slave SQL thread in SHOW PROCESSLIST. This would be because the slave is executing statements that are one hour old.

Q: How do I force the master to block updates until the slave catches up?

A: Use the following procedure:

  1. On the master, execute these statements:
    mysql> FLUSH TABLES WITH READ LOCK;
    mysql> SHOW MASTER STATUS;
    
    Record the log name and the offset from the output of the SHOW statement. These are the replication coordinates.
  2. On the slave, issue the following statement, where the arguments to the MASTER_POS_WAIT() function are the replication coordinate values obtained in the previous step:
    mysql> SELECT MASTER_POS_WAIT('log_name', log_offset);
    
    The SELECT statement will block until the slave reaches the specified log file and offset. At that point, the slave will be in sync with the master and the statement will return.
  3. On the master, issue the following statement to allow the master to begin processing updates again:
    mysql> UNLOCK TABLES;
    

Q: What issues should I be aware of when setting up two-way replication?

A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus, when the update of client A makes it to co-master 2, it will produce tables that are different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. This means that you should not co-chain two servers in a two-way replication relationship unless you are sure that your updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You must also realize that two-way replication actually does not improve performance very much (if at all), as far as updates are concerned. Both servers need to do the same number of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. Even this benefit might be offset by network delays.

Q: How can I use replication to improve performance of my system?

A: You should set up one server as the master and direct all writes to it. Then configure as many slaves as you have the budget and rackspace for, and distribute the reads among the master and the slaves. You can also start the slaves with the --skip-innodb, --skip-bdb, --low-priority-updates, and --delay-key-write=ALL options to get speed improvements on the slave end. In this case, the slave will use non-transactional MyISAM tables instead of InnoDB and BDB tables to get more speed.

Q: What should I do to prepare client code in my own applications to use performance-enhancing replication?

A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with a replicated setup should be very smooth and easy. Just change the implementation of your database access to send all writes to the master, and to send reads to either the master or a slave. If your code does not have this level of abstraction, setting up a replicated system will give you the opportunity and motivation to it clean up. You should start by creating a wrapper library or module with the following functions:

safe_ in each function name means that the function will take care of handling all the error conditions. You can use different names for the functions. The important thing is to have a unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.

You should then convert your client code to use the wrapper library. This may be a painful and scary process at first, but it will pay off in the long run. All applications that use the approach just described will be able to take advantage of a master/slave configuration, even one involving multiple slaves. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions; for example, to log how long each statement took, or which statement among your many thousands gave you an error.

If you have written a lot of code already, you may want to automate the conversion task by using the replace utility that comes with standard MySQL distributions, or just write your own conversion script. Ideally, your code already uses consistent programming style conventions. If not, then you are probably better off rewriting it anyway, or at least going through and manually regularizing it to use a consistent style.

Q: When and how much can MySQL replication improve the performance of my system?

A: MySQL replication is most beneficial for a system with frequent reads and infrequent writes. In theory, by using a single-master/multiple-slave setup, you can scale the system by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and to determine empirically by benchmarking the relationship between the throughput for reads (reads per second, or max_reads) and for writes (max_writes) on a typical master and a typical slave. The example here shows a rather simplified calculation of what you can get with replication for a hypothetical system.

Let's say that system load consists of 10% writes and 90% reads, and we have determined by benchmarking that max_reads is 1200 - 2 * max_writes. In other words, the system can do 1,200 reads per second with no writes, the average write is twice as slow as the average read, and the relationship is linear. Let us suppose that the master and each slave have the same capacity, and that we have one master and N slaves. Then we have for each server (master or slave):

reads = 1200 - 2 * writes

reads = 9 * writes / (N + 1) (reads are split, but writes go to all servers)

9 * writes / (N + 1) + 2 * writes = 1200

writes = 1200 / (2 + 9/(N+1))

The last equation indicates that the maximum number of writes for N slaves, given a maximum possible read rate of 1,200 per minute and a ratio of nine reads per write.

This analysis yields the following conclusions:

Note that these computations assume infinite network bandwidth and neglect several other factors that could turn out to be significant on your system. In many cases, you may not be able to perform a computation similar to the just shown that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decide whether and how much replication will improve the performance of your system:

Q: How can I use replication to provide redundancy/high availability?

A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see whether it is up. Then instruct your applications and the slaves to change master in case of failure. Some suggestions:

We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.

6.10 Troubleshooting Replication

If you have followed the instructions, and your replication setup is not working, first check the following:

6.11 Reporting Replication Bugs

When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to send us a bug report. We need to get as much information as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report.

If you have a repeatable test case that demonstrates the bug, please enter it into our bugs database at http://bugs.mysql.com/. If you have a phantom problem (one that you cannot duplicate ``at will''), use the following procedure:

  1. Verify that no user error is involved. For example, if you update the slave outside of the slave thread, the data will go out of sync, and you can have unique key violations on updates. In this case, the slave thread will stop and wait for you to clean up the tables manually to bring them in sync. This is not a replication problem. It is a problem of outside interference that causes replication to fail.
  2. Run the slave with the --log-slave-updates and --log-bin options. They will cause the slave to log the updates that it receives from the master into its own binary logs.
  3. Save all evidence before resetting the replication state. If we have no information or only sketchy information, it becomes difficult or impossible for us to track down the problem. The evidence you should collect is:
  4. Use mysqlbinlog to examine the binary logs. The following should be helpful to find the trouble query, for example:
    shell> mysqlbinlog -j pos_from_slave_status \
               /path/to/log_from_slave_status | head
    

Once you have collected the evidence for the phantom problem, try hard to isolate it into a separate test case first. Then enter the problem into our bugs database at http://bugs.mysql.com/ with as much information as possible.


Go to the first, previous, next, last section, table of contents.