How Percona XtraBackup Works

Percona XtraBackup is based on InnoDB‘s crash-recovery functionality. It copies your InnoDB data files, which results in data that is internally inconsistent; but then it performs crash recovery on the files to make them a consistent, usable database again.

This works because InnoDB maintains a redo log, also called the transaction log. This contains a record of every change to InnoDB data. When InnoDB starts, it inspects the data files and the transaction log, and performs two steps. It applies committed transaction log entries to the data files, and it performs an undo operation on any transactions that modified data but did not commit.

Percona XtraBackup works by remembering the log sequence number (LSN) when it starts, and then copying away the data files. It takes some time to do this, so if the files are changing, then they reflect the state of the database at different points in time. At the same time, Percona XtraBackup runs a background process that watches the transaction log files, and copies changes from it. Percona XtraBackup needs to do this continually because the transaction logs are written in a round-robin fashion, and can be reused after a while. Percona XtraBackup needs the transaction log records for every change to the data files since it began execution.

Percona XtraBackup uses Backup locks where available as a lightweight alternative to FLUSH TABLES WITH READ LOCK. This feature is available in Percona Server 5.6+. MySQL 8.0 allows acquiring an instance level backup lock via the LOCK INSTANCE FOR BACKUP statement.

Locking is only done for MyISAM and other non-InnoDB tables after Percona XtraBackup finishes backing up all InnoDB/XtraDB data and logs. Percona XtraBackup uses this automatically to copy non-InnoDB data to avoid blocking DML queries that modify InnoDB tables.

Important

To use effectively either LOCK INSTANCE FOR BACKUP or LOCK TABLES FOR BACKUP, the BACKUP_ADMIN privilege is needed in order to query performance_schema.log_status.

xtrabackup tries to avoid backup locks and FLUSH TABLES WITH READ LOCK when the instance contains only InnoDB tables. In this case, xtrabackup obtains binary log coordinates from performance_schema.log_status. FLUSH TABLES WITH READ LOCK is still required in MySQL 8.0 when xtrabackup is started with the --slave-info. The log_status table in Percona Server 8.0 is extended to include the relay log coordinates, so no locks are needed even with the --slave-info option.

See also

MySQL Documentation: More information about LOCK INSTANCE FOR BACKUP
https://dev.mysql.com/doc/refman/8.0/en/lock-instance-for-backup.html

When backup locks are supported by the server, xtrabackup first copies InnoDB data, runs the LOCK TABLES FOR BACKUP and then copies the MyISAM tables. Once this is done, the backup of the files will begin. It will backup .frm, .MRG, .MYD, .MYI, .ARM, .ARZ, .CSM, .CSV, .sdi and .par files.

After that xtrabackup will use LOCK BINLOG FOR BACKUP to block all operations that might change either binary log position or Exec_Master_Log_Pos or Exec_Gtid_Set (i.e. master binary log coordinates corresponding to the current SQL thread state on a replication slave) as reported by SHOW MASTER/SLAVE STATUS. xtrabackup will then finish copying the REDO log files and fetch the binary log coordinates. After this is completed xtrabackup will unlock the binary log and tables.

Finally, the binary log position will be printed to STDERR and xtrabackup will exit returning 0 if all went OK.

Note that the STDERR of xtrabackup is not written in any file. You will have to redirect it to a file, e.g., xtrabackup OPTIONS 2> backupout.log.

It will also create the following files in the directory of the backup.

During the prepare phase, Percona XtraBackup performs crash recovery against the copied data files, using the copied transaction log file. After this is done, the database is ready to restore and use.

The backed-up MyISAM and InnoDB tables will be eventually consistent with each other, because after the prepare (recovery) process, InnoDB‘s data is rolled forward to the point at which the backup completed, not rolled back to the point at which it started. This point in time matches where the FLUSH TABLES WITH READ LOCK was taken, so the MyISAM data and the prepared InnoDB data are in sync.

The xtrabackup offers many features not mentioned in the preceding explanation. The functionality of each tool is explained in more detail further in this manual. In brief, though, the tools enable you to do operations such as streaming and incremental backups with various combinations of copying the data files, copying the log files, and applying the logs to the data.

Restoring a backup

To restore a backup with xtrabackup you can use the --copy-back or --move-back options.

xtrabackup will read from the my.cnf the variables datadir, innodb_data_home_dir, innodb_data_file_path, innodb_log_group_home_dir and check that the directories exist.

It will copy the MyISAM tables, indexes, etc. (.MRG, .MYD, .MYI, .ARM, .ARZ, .CSM, .CSV, .sdi, and par files) first, InnoDB tables and indexes next and the log files at last. It will preserve file’s attributes when copying them, you may have to change the files’ ownership to mysql before starting the database server, as they will be owned by the user who created the backup.

Alternatively, the --move-back option may be used to restore a backup. This option is similar to --copy-back with the only difference that instead of copying files it moves them to their target locations. As this option removes backup files, it must be used with caution. It is useful in cases when there is not enough free disk space to hold both data files and their backup copies.