How Data Archive backup verification works

By default, the Data Archive backup task performs an incremental backup of Data Archive files.

The PI Backup subsystem attempts to maintain a consistent backup without corrupt or partially copied files in the Data Archive backup directory. It does this by temporarily copying files to a precommit directory before moving the files to their final destination. This precommit directory is a subdirectory under the Data Archive backup directory. If the backup is aborted, if the PI Backup subsystem crashes, or if the files in the precommit directory do not pass verification, then the files in the precommit directory are not moved to their final destination. Therefore, the last good backup should always be available.

The backup process works like this:

1. The files corresponding to the components selected for backup are copied to a precommit directory.

2. The primary archive and the files that correspond to the base and snapshot subsystems are verified in the precommit directory.

3. If verification passes, the files are moved to the backup directory. Any files with the same name that already exist in the backup directory are renamed before the move operation.

4. If all move operations are successful, the renamed files are deleted.

If the backup fails verification, the files are left in the precommit directory, and the reason for the failed backup is written to a pibackupverify_*.log file in the Data Archive backup directory. If successive backups fail verification, files will start accumulating in the precommit directory. After the next successful backup, all files are copied to their final destination.

The following table shows commands that the PI Backup subsystem spawns to perform the verification.

Although only the primary archive is verified by default, the number of archives to be verified can be set with the BackupVerification_NumArch tuning parameter. Alternatively, all verification can be suppressed by setting the BackupVerification tuning parameter to 0.

Although the last good backup should not be corrupt, it is still imperative to backup the Data Archive backup directory, preferably with third-party backup software. For example, if you accidentally delete a PI point and subsequently perform a backup, the PI point is deleted in the last good backup as well. To retrieve the deleted PI point, you might need to restore a previous backup. If you are not keeping multiple backups of your Data Archive backup directory, there will be no means to do this restore.