[libvirt] [RFC]: Snapshot API v3

Daniel Veillard veillard at redhat.com
Wed Mar 31 08:04:26 UTC 2010

On Tue, Mar 30, 2010 at 06:39:02PM -0400, Chris Lalancette wrote:
> On 03/30/2010 05:26 PM, Eric Blake wrote:
> > On 03/30/2010 02:52 PM, Matthias Bolte wrote:
> >> Yep, if a domain it not running you'll just get a disk snapshot
> >> without a memory snapshot.
> > 
> > Is taking a halted disk snapshot something we might want to allow with a
> > flag, though?  For that matter, is this API useful for taking a disk
> > snapshot and disregarding a memory snapshot even of a running VM?
> > 
> The problem with disgregarding a memory snapshot of a running guest is that it
> is very easy to get inconsistent snapshots; that is, the guest could
> have data cached in memory that is not on disk when you snapshot, and then you
> have completely inconsistent results on disk.
> ESX allows you to do a disk only snapshot of a running guest, but they cheat;
> they also have a way to quiesce the guest (force writes), thus ensuring a
> consistent snapshot.
> Virtualbox doesn't give you a choice in the matter; if the guest is off when
> you take a snapshot, you get a disk snapshot, and if it's on when you take a
> snapshot, you get a disk+memory snapshot.
> Qemu allows both usages, although it must have the consistency problems I
> mentioned above for a disk-only snapshot on a running guest.
> For now, I think we can stick with the disk-only semantic for shutoff guests,
> and the disk+memory semantic for running guests.  If it turns out that there
> is a need for a disk-only semantic for running guests, this should be easy
> to add later via a flag to virDomainSnapshotCreateXML.

  yes, that sounds a reasonable approach to me.


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