FreeBSD filesystem snapshots

FreeBSD filesystem snapshots

I’ve been looking at filesystem snapshots on FreeBSD lately and I have to say I wasn’t too impressed. Yes, the functionality is there, but the interface sucks. For UFS you have to create a snapshot, which takes a while, and which appears as a file within the filesystem you’re snapshotting. Then you have to use an md device to mount it. ZFS is easier, but different. What we need is a simple and standard interface to both.

Fortunately I discovered Ralf Engelschall’s snapshot management utilities. He’s written a couple of tools that make creating and managing snapshots really easy.

Using cron one can schedule creation of hourly, daily and weekly snapshots for filesystems. You can specify that you want 3 hourly snapshots, 2 daily and 1 weekly for a given filesystem. The system takes care of everything else. And by using amd (the FreeBSD automounter) these snapshots can automatically be made available through a given mount point. It couldn’t get any easier!

Well, actually, it could get slightly easier. Currently these tools are available from Ralf’s site or from the FreeBSD ports collection. It strikes me that something as useful and fundamental as this should really make its way in to the base system.

Update: Whilst this worked well in testing, once I applied it to my server with a large (approx, but not over, 2TB) filesystem it hung the machine completely, even on the console. I haven’t investigated further yet.

Update 2: Well, I have to say I’m disappointed. Ralf’s scripts worked great, but the snapshotting of large UFS filesystems in FreeBSD is as good as broken. When it takes hours to create a snapshot, locking out the filesystem (and maybe even the machine) for hours, it might as well not be there. There seems to be an attitude of “just accept it”, which I’m not impressed about either. Roll on ZFS… (yes – I know it’s there now, but I’d like it to mature just a little more 🙂 ).

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