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If I implement a brand new GFS backup rotation, where M-Th I'm running incremental backups, and on Friday I run a Full backup, how will the first incremental backup that runs on Monday know what to backup? My understanding is that the incremental (or diffs) are to run after the full runs...that is, they backup what has changed since the last full backup.

I've been reading up on the GFS rotation, and every chart I look at, and explanation that I read, has it with the incrementals (or diffs) running during the week, with the full running on Friday or Saturday. This seems backwards to me, in terms of grouping tapes for weekly sets. Shouldn't each set of weekly tapes you keep have the full backup leading, with the incrementals following???

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3 Answers 3

The full is "leading". You're think of the week starting on a Monday. In the backup's "mind", the week starts on Friday or Saturday. Your weekly sets would be all the tapes from the full through the last incremental or differential before the next full. Don't think about "calendar weeks", think about "backup weeks".

It doesn't matter which day of the week you run the full on-- you'll still start a restore by bringing back the last full and then the last differential (or all incrementals) taken after that full.

Typically, Friday or Saturday is chosen to give the longest window of non-business hours time to take the full backup. If you have a better day of the week for the full, use it.

BTW: If you can get away with differential backups use them instead of incrementals. You have much less risk exposure.

Edit: An incremental backup contains only the changes since the last full OR incremental. A differential backup contains all the changes since the last full. Assuming you're piling a week's worth of incrementals or differentials on the same tape cartridge, they're functionally very close to equivalent.

If, however, you're using a different tape cartridge each day for the incremental, you have a dependence on each of those cartridges holding the incremental backups to work for a successful restore. With a differential backup scenario using a different cartridge each day, only the last full and differential cartridges are needed for a restore.

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"BTW: If you can get away with differential backups use them instead of incrementals. You have much less risk exposure." Can you explain why this is? –  Citizen Chin Jun 19 '09 at 17:31

Our full backups run about 24 hours including verify, so we start them Friday night. As others have mentioned, differentials are better in that the worst case is you only have to restore twice, once to get things from the full, once from the last differential. In our case, fulls take 2 tapes (LTO3) (first we do disk-to-disk, then another copy goes onto tape) and we put the 4 diffs only on disk.

I've never used a strict GFS rotation, we always have at least 5 weeks of fulls, usually more now that tape costs have come down. Twice a year - once at the end of the fiscal year, once in the middle of the year - we take a set of full backup tapes and put them aside to be kept for "forever."

(We actually still have some TK70s and 8mm analog tapes from our old VAXes sitting in a drawer, and we even still have those VAXes, but there's no way we'd ever take the time to even try to boot them up and restore anything. We also have a ton of DAT tapes from our NT 3.51 and 4.0 days, but no more working drives to use them on if we wanted to.)

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You have to run a full first. Or your first incremental will basically be a full anyhow but a very slow one.

It helps if you think of your full backup as the first tape in your week, and the incremental backups as later in the week. So your week ends Friday at 5pm and your new week starts Friday at 5:01pm. So your Full Backups that run over the weekend are really the start of the next week.

The reason the full backups run Friday/Saturday/Sunday is they take a lot longer than do the incrementals.

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