Re: [Xen-users] snapshots on xen

[Javier Guerra Giraldez <javier@xxxxxxxxxxx>]

On Mon, Apr 12, 2010 at 4:01 AM, Pasi KÃrkkÃinen <pasik@xxxxxx> wrote:
> You'll get fsck when you restore that kind of snapshot and start the guest.
> And some applications might be in a bad state.

even worse:  if you mount the filesystem to perform the backup while
the VM is suspended to disk (by xm save), it would automatically do a
journal replay/revert/whatever.  suppose it goes smoothly and you now
have a consistent FS (usually it wil); now you resume the VM and....
it would complain, most likely crash, and maybe corrupt the filesystem

why? simple, you changed the filesystem (by mounting it) while it was
sleeping.  from the point of view of the VM, in a blink, and without
any warning, several critical disk structures were changed and the're
no longer consistent with the memory state.

even if you mount the filesystem as R/O, most would perform a journal
replay on mounting.  there's usually an obscure mount option to block
it, or you might set the block device as R/O, but then you don't have
a consistent FS to backup.  Better save the replay to a different
medium, like a COW file, or an LVM R/W snapshot.

and we're back at the beginning....

and still it's not a full backup, since the guest didn't have any
chance to flush it's buffers or userspace caches (like all DBs do)



really, just do backups from inside the guest.



-- 
Javier

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