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14.3.3 The Slave Status Files »
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14.3.2. The Slave Relay Log

The relay log, like the binary log, consists of a set of numbered files containing events that describe database changes, and an index file that contains the names of all used relay log files.

The term “relay log file” generally denotes an individual numbered file containing database events. The term “relay log” collectively denotes the set of numbered relay log files plus the index file.

Relay log files have the same format as binary log files and can be read using mysqlbinlog (see Section 4.6.6, “mysqlbinlog — Utility for Processing Binary Log Files”).

By default, relay log file names have the form host_name-relay-bin.nnnnnn in the data directory, 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 uses an index file to track the relay log files currently in use. The default relay log index file name is host_name-relay-bin.index in the data directory.

The default relay log file and relay log index file names can be overridden with, respectively, the --relay-log and --relay-log-index server options (see Section 14.8, “Replication and Binary Logging Options and Variables”).

If a slave uses the default host-based relay log file names, changing a slave's host name after replication has been set up 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 (see Bug#2122). If you anticipate that a slave's host name might change in the future (for example, if networking is set up on the slave such that its host name can be modified via DHCP), you can avoid this issue entirely by using the --relay-log and --relay-log-index options to specify relay log file names explicitly when you initially set up the slave. This will make the names independent of server host name changes.

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. (This creates a new relay log only as of MySQL 4.0.14.)

  • When the size of the current relay log file becomes “too large,” determined as follows:

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, as of MySQL 4.0.14, FLUSH LOGS rotates relay logs, which influences when the SQL thread deletes them.