After creating a backup, the data is not ready to be restored. There might be uncommitted transactions to be undone or transactions in the logs to be replayed. Doing those pending operations will make the data files consistent and it is the purpose of the prepare stage. Once this has been done, the data is ready to be used.
To prepare a backup with innobackupex you have to use the --apply-log and the path to the backup directory as an argument:
$ innobackupex --apply-log /path/to/BACKUP-DIR
and check the last line of the output for a confirmation on the process:
150806 01:01:57 InnoDB: Shutdown completed; log sequence number 1609228 150806 01:01:57 innobackupex: completed OK!
If it succeeded, innobackupex performed all operations needed, leaving the data ready to use immediately.
innobackupex started the prepare process by reading the configuration from the backup-my.cnf file in the backup directory.
After that, innobackupex replayed the committed transactions in the log files (some transactions could have been done while the backup was being done) and rolled back the uncommitted ones. Once this is done, all the information lay in the tablespace (the InnoDB files), and the log files are re-created.
Note that this preparation is not suited for incremental backups. If you perform it on the base of an incremental backup, you will not be able to “add” the increments. See Incremental Backups with innobackupex.
The preparing process can be speed up by using more memory in it. It depends on the free or available RAM on your system, it defaults to 100MB. In general, the more memory available to the process, the better. The amount of memory used in the process can be specified by multiples of bytes:
$ innobackupex --apply-log --use-memory=4G /path/to/BACKUP-DIR
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