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We're running Time Machine on our SL server and recently purchased a Drobo S with the intent to perform regular local backups of the whole server. However, it's not really useful for us to keep backups older than a couple of weeks or a maximum of one month. We just don't need that kind of redundancy and really it's the last week or two that is interesting in case of a catastrophy.

However, I can't seem to find any way of limiting the age of the Time Machine backups to correspond with our demands. To me it seems like it simply eats all available space and if the space is depleted THEN it starts to remove old versions. This seems woefully inefficient, especially considering that Time Machine doesn't seem to store deltas but actual whole copies of the modified files which in itself is inefficient (although, it might make sense).

So in short, is it possible to limit the age of the time machine backups?

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FWIW, Time Machine does not make a complete backup each time. It still backups only the deltas. I recommend checking out the question on Server Fault: What Is Time Machine Doing? serverfault.com/questions/9422/what-is-time-machine-doing – Chealion Dec 16 '09 at 0:02
I think you may have misread my statement or I misunderstood the information in your link. I don't mean that Time Machine stores a complete copy of the drive whenever some file(s) changes, but rather that it stores a copy of the file(s) in question. So if you have a 1gb PSD-file where a few bytes here and there changes, then it'll actually make a complete backup of the whole file instead of just storing the changes and computing the file using some baseline copy (like any decent versioning system does these days). I may be wrong though. – macke Jun 3 '10 at 10:31
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Yes, Drobo has a script to do that.

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The script worked for backing up, but I was never able to use "Enter Time Machine" to restore. I would have to dig around in the mounted sparseimage manually. Which sucked. – The Doctor What Jun 26 '10 at 4:53
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There is no built-in method to restrict the amount of space that Time Machine will consume, but I would like to just point out that Backup in-depth, is not a bad thing.

Yes, it does incremental backups, but that actually is quite smart, since the user can just transverse to the directory and drag & drop the backup file for a restore, instead of being forced to use a particular application to restore...

But that's not answering your question is it? In my experience, in a network environment, Time machine's self management of disk space is reasonable, but when there is more than a few people backing up over the network it can become annoying...

But at this time, there is no way (that I know of) in a stock environment to self manage it.. The Drobos app sound interesting, but unless it assigns quota's, I don't see how it manages it.

Part of the issue, is the hard linking nature of time machine, if you delete a file or folder, it may actually be a hard link to a previous backup, or used by a newer backup... You would have to check the link count before deleting it. That's why time machine can be so efficient in space management. There's only one copy of the file or folder, in the same location...

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Good post, but not an answer. Thanks anyhow though, gave you an upvote at least! – macke Jun 3 '10 at 10:32
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up vote 1 down vote

While there is no explicit way to limit the actual age of a Time Machine backup it is possible to limit the amount of space is used on an external hard drive by Time Machine by forcing Time Machine to use a size limited sparseimage (Mac OS X Hints Instrucitons).

Time Machine by design won't start removing older backups until it has run out of available space for it's next delta backup.

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Drobo has a kb article for that: http://support.datarobotics.com/app/answers/detail/a_id/119

I found that I had to eject my drobo, disconnect it from my MacBook Pro and then power it off (after the power light went orange).

Once I powered it up and reconnected it, the Drobo Manager showed the correct space usage and the blue lights on the front adjusted themselves after a few minutes.

Unfortunately, it looks the the Drobo Manager's "Advanced Tools" don't understand the multiple-volumes-on-a-drobo, so you can rename it via the manager, etc.

Allegedly, the extra space from new disks will be added to the second volume, but I haven't tested it yet. :-/

Ciao!

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