Skip navigation links

Additional languages
15.4.2.2 The Slave Status Files »
Section Navigation      [Toggle]

15.4.2.1. The Slave Relay Log

By default, relay logs filenames have 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. The slave uses an index file to track the relay log files currently in use. The default relay log index filename is host_name-relay-bin.index. By default, the slave server creates relay log files in its data directory.

The default filenames for relay logs and relay log index files can be overridden with, respectively, the --relay-log and --relay-log-index server options (see Section 15.1.4, “Replication Options and Variables”). For this reason, changing a replication slave's hostname can cause replication to fail with the errors Failed to open the relay log and Could not find target log during relay log initialization. This is a known issue which we intend to fix in a future MySQL release (see Bug#2122). If you anticipate that a slave's hostname may change in the future (for example, if networking is set up on the slave such that its hostname can be modified via DHCP), then you can use these options to prevent this problem from occurring. However, if you encounter this issue, one way to work around it is to stop the slave server, prepend the contents of the old relay log index file to the new one, then restart the slave. On a Unix system, this can be done as shown here, where new_host_name is the new hostname and old_host_name is the old one: 

shell< cat new_host_name-relay-bin.index >> old_host_name-relay-bin.index

shell< mv old_host_name-relay-bin.index new_host_name-relay-bin.index

Relay logs have the same format as binary logs and can be read using mysqlbinlog. The SQL thread automatically deletes each relay log file as soon as it has executed all events in the file and no longer needs it. There is no explicit mechanism for deleting relay logs because the SQL thread takes care of doing so. However, FLUSH LOGS rotates relay logs, which influences when the SQL thread deletes them.

A slave server creates a new relay log file under the following conditions:

  • Each time the I/O thread starts.

  • When the logs are flushed; for example, with FLUSH LOGS or mysqladmin flush-logs.

  • When the size of the current relay log file becomes too large. The meaning of “too large” is determined as follows:

    • If the value of max_relay_log_size is greater than 0, that is the maximum relay log file size.

    • If the value of max_relay_log_size is 0, max_binlog_size determines the maximum relay log file size.