To guarantee the integrity of the files that are copied, backing up the raw data files on your MySQL replication slave should take place while your slave server is shut down. If the MySQL server is still running, background tasks may still be updating the database files, particularly those involving storage engines with background processes such as InnoDB. With InnoDB, these problems should be resolved during crash recovery, but since the slave server can be shut down during the backup process without affecting the execution of the master it makes sense to take advantage of this capability.

To shut down the server and back up the files:

Normally you should back up the entire data directory for the slave MySQL server. If you want to be able to restore the data and operate as a slave (for example, in the event of failure of the slave), in addition to the data, you need to have the master info repository and relay log info repository, and the relay log files. These items are needed to resume replication after you restore the slave's data. If tables have been used for the master info and relay log info repositories (see Section 17.2.4, “Replication Relay and Status Logs”), which is the default in MySQL 8.0, these tables are backed up along with the data directory. If files have been used for the repositories, you must back these up separately. The relay log files must also be backed up separately if they have been placed in a different location to the data directory.

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 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 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.