The xtrabackup binary integrates with information that InnoDB stores in its transaction log about the corresponding binary log position for committed transactions. This enables it to print out the binary log position to which a backup corresponds, so you can use it to set up new replication slaves or perform point-in-time recovery.
You can find the binary log position corresponding to a backup performing the --prepare process. If your backup is from a server with binary logging enabled, xtrabackup will create a file named xtrabackup_binlog_info in the target directory. This file contains the binary log file name and position of the exact point in the binary log to which the prepared backup corresponds.
You will also see output similar to the following during the prepare stage:
InnoDB: Last MySQL binlog file position 0 3252710, file name ./mysql-bin.000001 ... snip ... [notice (again)] If you use binary log and don't use any hack of group commit, the binary log position seems to be: InnoDB: Last MySQL binlog file position 0 3252710, file name ./mysql-bin.000001
If other storage engines are used (i.e. MyISAM), you should use the xtrabackup_binlog_info file to retrieve the position.
The message about hacking group commit refers to an early implementation of emulated group commit in Percona Server.
To perform a point-in-time recovery from an xtrabackup backup, you should prepare and restore the backup, and then replay binary logs from the point shown in the xtrabackup_binlog_info file.
A more detailed procedure is found here (with innobackupex).
To set up a new replica, you should prepare the backup, and restore it to the data directory of your new replication slave. Then in your CHANGE MASTER TO command, use the binary log filename and position shown in the xtrabackup_binlog_info file to start replication.
A more detailed procedure is found in How to setup a slave for replication in 6 simple steps with Percona XtraBackup.
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