The relay log is a set of log files created by a slave during replication.

It's the same format as the binary log, containing a record of events that affect the data or structure; thus, mysqlbinlog can be used to display its contents. It consists of a set of relay log files and an index file containing a list of all relay log files.

Events are read from the master's binary log and written to the slave's relay log. They are then performed on the slave. Old relay log files are automatically removed once they are no longer needed.

Creating relay log files

New relay log files are created by the slave at the following times:

• when the IO thread starts
• when the logs are flushed, with FLUSH LOGS or mysqladmin flush-logs.
• when the maximum size, determined by the max_relay_log_size system variable, has been reached

Relay log names

By default, the relay log will be given a name host_name-relay-bin.nnnnnn, with host_name referring to the server's host name, and #nnnnnn the sequence number.

This will cause problems if the slave's host name changes, returning the error Failed to open the relay log and Could not find target log during relay log initialization. To prevent this, you can specify the relay log file name by setting the relay_log and relay_log_index system variables.

If you need to overcome this issue while replication is already underway,you can stop the slave, prepend the old relay log index file to the new relay log index file, and restart the slave.

For example:

shell> cat NEW_relay_log_name.index >> OLD_relay_log_name.index
shell> mv NEW_relay_log_name.index OLD_relay_log_name.index

Viewing relay logs

The SHOW RELAYLOG EVENTS shows events in the relay log, and, since relay log files are the same format as binary log files, they can be read with the mysqlbinlog utility.

Removing old relay logs

Old relay logs are automatically removed once all events have been implemented on the slave, and the relay log file is no longer needed. This behavior can be changed by adjusting the relay_log_purge system variable from its default of 1 to 0, in which case the relay logs will be left on the server.

If the relay logs are taking up too much space on the slave, the relay_log_space_limit system variable can be set to limit the size. The IO thread will stop until the SQL thread has cleared the backlog. By default there is no limit.