question

Fatherjack avatar image
Fatherjack asked

Differential backup size prediction

I currenly have a backup schedule of

Full, Diff, Diff, Diff, Diff, Full, Diff, Diff, Diff, Diff

If the sizes for these are

1GB, 2Mb, 4Mb, 6Mb, 8Mb, 1GB, 2Mb, 4Mb, 6Mb, 8Mb

how would it change if I stop doing the second full backup? Would I get

1GB, 2Mb, 4Mb, 6Mb, 8Mb, 10Mb, 12Mb, 14Mb, 16Mb

I'm weighing up the worth of the Full backup hitting the disks so hard...

backupdifferential-backup
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TimothyAWiseman avatar image
TimothyAWiseman answered

Having not actually tested it, my understanding is that yes, that is roughly what you would get. As you continued accumulating more changes the differential would continue growing, and at roughly the same rate as it was before, until the next full backup is done.

Whether the extra full backup is worth it or not depends on how hard it is hitting the disks and if you can fit the full backup inside a maintenance window so its affect on users is minimal or not. As a general rule though, the extra full backup is worth it. I have seen cases where the differential backup can grow very large and time consuming.

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Steve Jones - Editor avatar image
Steve Jones - Editor answered

I like Timothy's answer, and the other thing I'll add is that you are increasing your risk if you remove the 2nd full. You are now depending on the first full backup for a longer period of time, which means more chance for an issue with that file, tape, etc.

Having more regular backups ensures that your risk is lower since each is valid for a shorter period of time.

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Nick Kavadias avatar image
Nick Kavadias answered

Paul wrote a script a while back that works out how many extents have changed since your last full backup, so instead of guesstimating how big your diffs are going to be you can know in advance & go for a full backup if it's over a certain percentage.

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Hi Nick, good to see you here as well as ServerFault! The Full file size is 3.5GB, the biggest Diff is 160Mb so if by the end of the day I have double that its still just 10% of the effort of the Full...
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sounds reasonable. You're backing up to a separate set of disks then data/log right?
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