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 mysqlbinlog — Utility for Processing Binary Log Files).
By default, relay log file names have the form
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
slave uses an index file to track the relay log files currently
in use. The default relay log index file name is
in the data directory.
The default relay log file and relay log index file names can be
overridden with, respectively, the
--relay-log-index server options
(see Section 2.6, “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
using DHCP), you can avoid this issue entirely by using the
--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.
If you encounter the issue after replication has already begun, 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, and then restart the slave. On a Unix system, this can be done as shown here:
shell> cat new_relay_log_name.index >> old_relay_log_name.index shell> mv old_relay_log_name.index new_relay_log_name.index
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 LOGSor mysqladmin flush-logs.
When the size of the current relay log file becomes “too large,” determined as follows:
The SQL thread automatically deletes each relay log file after
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.