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Re: [Linux-cluster] LVM Snapshots

On Wed, 6 Feb 2008, Jakub Suchy wrote:

i am currently designing a cluster of XEN machines, having two nodes and
one shared storage (SAS). We have Recovery Point Objective of 15
minutes, so I am thinking about putting XEN machine on a LVM partition
and making a snapshot of this partition every 15 minutes.
Do anybody know any caveats of this? I wonder how fast would this be,
i am afraid that every 15 minutes is a little bit too often. XEN machine
will be Sybase server.

The caveat is that snapshot != backup. Not even close.
For a start, the underlying FS will be about as consistent as you'd expect it to be if you just power cycled the machine while it's working. A journalled FS will work around the need to fsck it, but your DB files (and in fact, any open files) are likely to end up not being internally consistent. You'd need to run check/repair on all your DB tables, for example. That may or may not be an acceptable solution for you, because it is not guaranteed that you will not lose any data.

Other than that, snapshotting frequency isn't a problem. It's just an undo log, not a complete copy. Bear in mind, though, that the more snapshots you concurrently have, the more times you multiply the writes, so the disk I/O preformance will degrade if you are running too many simultaneously.


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