Identical tape backups, but one is 3 times as big

[pshute]

We do a weekly backup to tape, and an additional monthly one, to separate media pools. Looking at the job histories, a typical monthly backup is about 3 times the size of a weekly one. It looks like the problem is that we're keeping 10 (daily) retention points, which typically includes 3 full backups, and they're all being backed up to tape by the monthly backup.

I've now ticked "Process latest full backup chain only", and it looks like this will reduce the data to be backed up to similar to what's the weekly backup is doing.

Can someone please explain why the weekly backup wasn't having the same difficulty? Does each backup job know which backup files have already been backed up to the that media pool?

[veremin]

Can someone please explain why the weekly backup wasn't having the same difficulty?

Most likely because it's copied already the previous full backups. Therefore, each time it gets executed, it copies only one, latest full backup.

In contrast, monthly job hasn't copied those fulls before, thus, when the time comes, it processes all of them. The said option should make a monthly job archive only one full, though.

Thanks.

[pshute]

Thanks for confirming that. Can you please tell me where in the documentation I can read about media pools keeping track of what they've backed up?

Comparing to other tape backup software like Backup Exec, they decide what to backup according to criteria like modification dates or files' archive attribute, and media pools have nothing to do with it.

[veremin]

Sure, this is controlled by retention settings. Meaning whatever information has been written to tapes a given media pool consists of, it will be preserved for the duration of specified time interval. During this period a tape job won't copy the same information again.

In your case, corresponding full backups have been archived previously. Those backups are still present on cassettes, being under given retention policy. Thus, when the tape job starts, it skips those files.

Thanks.