[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapsh

From: Daniel P . Berrangé
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI
Date: Tue, 13 Feb 2018 14:58:44 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

On Tue, Feb 13, 2018 at 03:43:10PM +0100, Kevin Wolf wrote:
> Am 13.02.2018 um 15:30 hat Roman Kagan geschrieben:
> > On Tue, Feb 13, 2018 at 11:50:24AM +0100, Kevin Wolf wrote:
> > > Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben:
> > > > Then you could just use the regular migrate QMP commands for loading
> > > > and saving snapshots.
> > > 
> > > Yes, you could. I think for a proper implementation you would want to do
> > > better, though. Live migration provides just a stream, but that's not
> > > really well suited for snapshots. When a RAM page is dirtied, you just
> > > want to overwrite the old version of it in a snapshot [...]
> > 
> > This means the point in time where the guest state is snapshotted is not
> > when the command is issued, but any unpredictable amount of time later.
> > 
> > I'm not sure this is what a user expects.
> I don't think it's necessarily a big problem as long as you set the
> expectations right, but good point anyway.
> > A better approach for the save part appears to be to stop the vcpus,
> > dump the device state, resume the vcpus, and save the memory contents
> > in the background, prioritizing the old copies of the pages that
> > change.
> So basically you would let the guest fault whenever it writes to a page
> that is not saved yet, and then save it first before you make the page
> writable again? Essentially blockdev-backup, except for RAM.

The page fault servicing will be delayed by however long it takes to
write the page to underling storage, which could be considerable with
non-SSD. So guest performance could be significantly impacted on slow
storage with high dirtying rate. On the flip side it gurantees a live
snapshot would complete in finite time which is good.

|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

reply via email to

[Prev in Thread] Current Thread [Next in Thread]