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Re: [Xen-users] A simple backup

Mark Williamson wrote:
Some investigation prompted by observations by users on this mailing list revealed that LVM can notify a filesystem *within the same VM* that it's going to be snapshotted so that it can put itself into a consistent state. I'd just like to add the plumbing so that you can tell a filesystem in another VM "You're going to get snapshotted by dom0. Make your filesystem consistent so the snapshot will be mountable."

You really need to go a little further than that.

The file system may be self-consistent but the data stored there may not be. For example, in my former job we used to run "omsuspend" to put the (mail) message store into a self-consistent state, that is, to commit all outstanding transactions on the message store so that a backup would come up cleanly. Of course, the message store in question is resiliant enough to cope with what amounts to a sudden power failure, but the you still have to run a consistency check to get everything OK. (That particular message store would have been much better with transaction logs, but that's not germane to the topic in question.)

The point is that it's not just the file system you need to worry about: applications need to get their house in order to prepare for the backup. This problem is largely solved if you do the backup from the guest in domU, but would clearly need some work to be done outside domU.

Taking a snapshot of both memory and disk image does work to a large extent, but in the case of this message store simply bringing up the old memory and disk image suddenly leaves the guest OS wondering what to do with all these network connections it used to have -- it will generally recover and clean-up its aborted transactions, but, for example, messages that were being sent _out_ at the time of the snapshot will be resent because the outgoing connection didn't acknowledge the message. Depending on what the message is this will vary from the merely annoying all the way through to the downright weird.

I would imagine that there are other application domains where restarting a transaction from the restored domU would have rather unpleasant side effects.


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