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How should Time Machine be setup?
Authored by: cmcd on Nov 13, '09 06:45:35AM

I've done additional analysis, with both TimeTracker and Finder/Path Finder. It does appear that Time Machine is just backing up the bands that have been modified since the last TM backup - as expected. The surprise for me is how much space the bands are taking up - again, I'm seeing about 400-500mb per daily backup. I suspect the issue is the 8mb band size. For example, for the few backups I examined, the number of bands was 50-70 - multiply by 8mb per band and I get my number. And 50-70 modified bands may correspond to about the number of emails I receive/send in a day (though it seems a bit high).

So, maybe the answer is to convert the sparsebundle file to use either smaller - or much larger - bands. Since email messages are usually small, maybe use 1mb bands instead of 8mb. Then, 70 bands will only be 70mb (except when I receive emails with large attachments). I think hdiutil can convert sparsebundle band size. Any thoughts? My problem is I'm dealing with 2 sparsebundles: the relatively small Microsoft Identities file, and the huge Time Machine backup file on the Time Capsule.



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How should Time Machine be setup?
Authored by: everkleer80 on Nov 13, '09 09:52:18AM

I think you're right on with changing the band size. But it sounds like the messages may be spread throughout the file rather than being grouped together (not sure about this) which means that multiple messages may or may not go into one band depending on how close together they are in the mailbox file. So if you increase the band size to, say, 16 MB, and have 10 new messages that are all < 1MB and are grouped together, then they may all go into the same band and you will only have to backup 16MB, but then again if the messages are spread apart in the file and each one goes into a different band, then you will be backing up 160MB. So I think smaller is definately the way to go here so no matter where in the data your messages are, you will be backing up closer to the amount of data that actually changed (maybe make the bands 1 or 2MB if you can?)

I'm not sure if you're saying there is any sort of problem with your TM sparsebundle or not or if your just commenting on the fact that we're dealing with two different sparsebundles, but anyway I don't think you should do anything to the TM bundle. I'm not sure why exacly TM backs up directly to the drive if it's local, and to a sparsebundle if it's a network drive, but either way I'd leave that one alone and just let TM manage it.



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How should Time Machine be setup?
Authored by: cmcd on Nov 16, '09 01:54:14PM
OK, now I'm using a sparsebundle file with 1MB bands instead of the default 8MB. It's soon after the change and I'm still monitoring, but the conversion appears to have the desired result - my backup is now in the 100MB range instead of 500MB range. Maybe 500KB bands would reduce the backup size further, I don't know. I used this command to convert the sparsebundle file:
hdiutil convert ~/Documents/Office_2008_backup.sparsebundle -format UDSB -tgtimagekey sparse-band-size=2048 -o ~/Documents/Office_2008_IdentitiesNEW.sparsebundle
where Office_2008_IdentitiesNEW.sparsebundle is the converted file with the 1MB bands. After the conversion, I created a new alias pointing to the converted file.
Edited on Nov 16, '09 01:55:08PM by cmcd


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How should Time Machine be setup?
Authored by: cmcd on Nov 17, '09 01:26:15PM

According to the man page for hdiutil, 1MB bands are the smallest allowed.



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