You can add another slave to an existing replication configuration without stopping the master. To do this, you can set up the new slave by copying the data directory of an existing slave, and giving the new slave a different server ID (which is user-specified) and server UUID (which is generated at startup).
To duplicate an existing slave:
Stop the existing slave and record the slave status information, particularly the master binary log file and relay log file positions. You can view the slave status either in the Performance Schema replication tables (see Performance Schema Replication Tables), or by issuing
SHOW SLAVE STATUSas follows:
mysql> STOP SLAVE; mysql> SHOW SLAVE STATUS\G
Shut down the existing slave:
shell> mysqladmin shutdown
Copy the data directory from the existing slave to the new slave, including the log files and relay log files. You can do this by creating an archive using tar or
WinZip, or by performing a direct copy using a tool such as cp or rsync.Important
Before copying, verify that all the files relating to the existing slave actually are stored in the data directory. For example, the
InnoDBsystem tablespace, undo tablespace, and redo log might be stored in an alternative location.
InnoDBtablespace files and file-per-table tablespaces might have been created in other directories. The binary logs and relay logs for the slave might be in their own directories outside the data directory. Check through the system variables that are set for the existing slave and look for any alternative paths that have been specified. If you find any, copy these directories over as well.
During copying, if files have been used for the master info and relay log info repositories (see Section 5.4, “Replication Relay and Status Logs”), ensure that you also copy these files from the existing slave to the new slave. If tables have been used for the repositories, which is the default from MySQL 8.0, the tables are in the data directory.
After copying, delete the
auto.cnffile from the copy of the data directory on the new slave, so that the new slave is started with a different generated server UUID. The server UUID must be unique.
A common problem that is encountered when adding new replication slaves is that the new slave fails with a series of warning and error messages like these:
071118 16:44:10 [Warning] Neither --relay-log nor --relay-log-index were used; so replication may break when this MySQL server acts as a slave and has his hostname changed!! Please use '--relay-log=new_slave_hostname-relay-bin' to avoid this problem. 071118 16:44:10 [ERROR] Failed to open the relay log './old_slave_hostname-relay-bin.003525' (relay_log_pos 22940879) 071118 16:44:10 [ERROR] Could not find target log during relay log initialization 071118 16:44:10 [ERROR] Failed to initialize the master info structure
This situation can occur if the
--relay-logoption is not specified, as the relay log files contain the host name as part of their file names. This is also true of the relay log index file if the
--relay-log-indexoption is not used. See Section 2.6, “Replication and Binary Logging Options and Variables”, for more information about these options.
To avoid this problem, use the same value for
--relay-logon the new slave that was used on the existing slave. If this option was not set explicitly on the existing slave, use
. If this is not possible, copy the existing slave's relay log index file to the new slave and set the
--relay-log-indexoption on the new slave to match what was used on the existing slave. If this option was not set explicitly on the existing slave, use
. Alternatively, if you have already tried to start the new slave after following the remaining steps in this section and have encountered errors like those described previously, then perform the following steps:
If you have not already done so, issue
STOP SLAVEon the new slave.
If you have already started the existing slave again, issue
STOP SLAVEon the existing slave as well.
Copy the contents of the existing slave's relay log index file into the new slave's relay log index file, making sure to overwrite any content already in the file.
Proceed with the remaining steps in this section.
When copying is complete, restart the existing slave.
On the new slave, edit the configuration and give the new slave a unique server ID (using the
server-idoption) that is not used by the master or any of the existing slaves.
Start the new slave server, specifying the
--skip-slave-startoption so that replication does not start yet. Use the Performance Schema replication tables or issue
SHOW SLAVE STATUSto confirm that the new slave has the correct settings when compared with the existing slave. Also display the server ID and server UUID and verify that these are correct and unique for the new slave.
Start the slave threads by issuing a
mysql> START SLAVE;
The new slave now uses the information in its master info repository to start the replication process.