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Re: [Xen-users] Perfectly consistent zero-downtime backup.

(Try to avoid top-posting)

On Thu, Apr 18, 2013 at 4:57 PM, Andrew Wells <agwells0714@xxxxxxxxx> wrote:
> If you do a dd of an lvm snapshot, doesn't that promise that no writes were
> taking place during that time? However I know that this type of backup is
> identical to pulling out the power, which might not be to bad in some
> circumstances depending on applications running and the volatility of the
> data, but is generally frowned upon.

I must have missed the LVM snapshot bit. I was referring to doing a dd
on a live, mounted partition (which is what I assumed the suggestion
was for). Doing this is a bad idea, as any file edits that occur after
the partition index sectors have been copied will leave the index out
of sync with the data. Thus, any edit can potentially corrupt your

Now, if it's an LVM snapshot that's not currently in use, then a dd
would be acceptable (though you may consider using something like
partclone, which can intelligently copy and compress most
filesystems). The other issue that you touch on is one that I don't
know enough about to answer. This post[1] leads me to believe that
snapshots should be atomic, but I don't know for sure. It seems like
it would depend on your filesystem. I also found a little bit on doing
[semi]automated LVM snapshots [2]. Maybe it will be of help.

[2] http://tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html


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