Re: [Xen-users] snapshots on xen

[Thomas Halinka <lists@xxxxxxxxx>]

Hi,

Am Montag, den 12.04.2010, 09:25 -0500 schrieb Javier Guerra Giraldez:
> 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)

Full ACK

> really, just do backups from inside the guest.

NACK - since Citrix has xe vm-snapshot :-)

Is there something like "xm snapshot" planned for OS-XEN?

i cant find anything @http://wiki.xensource.com/xenwiki/XenRoadMap

Best Regards,

Thomas



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