A slave replication server creates two small status files. By
default, these files are named master.info
and relay-log.info
and created in the data
directory. Their names and locations can be changed by using the
--master-info-file
and
--relay-log-info-file
options
(see Section 14.8, “Replication and Binary Logging Options and Variables”).
The two status files contain information like that shown in the
output of the SHOW SLAVE STATUS
statement, which is discussed in
Section 12.5.2, “SQL Statements for Controlling Slave Servers”. Because the status
files are stored on disk, they survive a slave server's
shutdown. The next time the slave starts up, it reads the two
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 should be protected
because it contains the password for connecting to the master.
See Section 5.4.2.1, “Administrator Guidelines for Password Security”.
The I/O thread updates the master.info
file. Before MySQL 4.1, the following table shows the
correspondence between the lines in the file and the columns
displayed by SHOW SLAVE STATUS
.
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 SQL thread updates the relay-log.info
file. The following table shows the correspondence between the
lines in the file and the columns displayed by
SHOW SLAVE STATUS
.
Line | Description |
1 | Relay_Log_File |
2 | Relay_Log_Pos |
3 | Relay_Master_Log_File |
4 | Exec_Master_Log_Pos |
The contents of the relay-log.info
file and
the states shown by the SHOW SLAVE
STATUS
statement might not match if the
relay-log.info
file has not been flushed to
disk. Ideally, you should only view
relay-log.info
on a slave that is offline
(that is, mysqld
is not running). For a
running system, SHOW SLAVE STATUS
should be used.
When you back up the slave's data, you should back up these two
status 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 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 location of this directory is
the value of the
--slave-load-tmpdir
option. If
the server was not started with that option, the directory
location is the value of the
tmpdir
system variable.
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