From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39662) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1elc3H-0006qf-2v for qemu-devel@nongnu.org; Tue, 13 Feb 2018 09:59:26 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1elc3F-0008La-St for qemu-devel@nongnu.org; Tue, 13 Feb 2018 09:59:23 -0500 Date: Tue, 13 Feb 2018 14:59:14 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20180213145913.GE2378@work-vm> References: <20180107122336.29333-1-richiejp@f-m.fm> <5cf19623-72ac-fb8b-2054-a60d42419ec6@redhat.com> <20180111130427.GG8326@redhat.com> <20180213105024.GC5083@localhost.localdomain> <20180213143001.GA2354@rkaganb.sw.ru> <20180213143615.GN573@redhat.com> <20180213144521.GI5083@localhost.localdomain> <20180213144838.GO573@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20180213144838.GO573@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Cc: Kevin Wolf , Roman Kagan , Richard Palethorpe , Qemu-block , quintela@redhat.com, qemu-devel@nongnu.org, armbru@redhat.com, Max Reitz , rpalethorpe@suse.com, Denis Plotnikov , Denis Lunev * Daniel P. Berrang=E9 (berrange@redhat.com) wrote: > On Tue, Feb 13, 2018 at 03:45:21PM +0100, Kevin Wolf wrote: > > Am 13.02.2018 um 15:36 hat Daniel P. Berrang=E9 geschrieben: > > > On Tue, Feb 13, 2018 at 05:30:02PM +0300, Roman Kagan wrote: > > > > 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. > > > > >=20 > > > > > Yes, you could. I think for a proper implementation you would w= ant 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, y= ou just > > > > > want to overwrite the old version of it in a snapshot [...] > > > >=20 > > > > 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. > > > >=20 > > > > I'm not sure this is what a user expects. > > > > > > > > A better approach for the save part appears to be to stop the vcp= us, > > > > dump the device state, resume the vcpus, and save the memory cont= ents in > > > > the background, prioritizing the old copies of the pages that cha= nge. > > > > No multiple copies of the same page would have to be saved so the= stream > > > > format would be fine. For the load part the usual inmigrate shou= ld > > > > work. > > >=20 > > > No, that's policy decision that doesn't matter from QMP pov. If the= mgmt > > > app wants the snapshot to be wrt to the initial time, it can simply > > > invoke the "stop" QMP command before doing the live migration and > > > "cont" afterwards. > >=20 > > That would be non-live. I think Roman means a live snapshot that save= s > > the state at the beginning of the operation. Basically the difference > > between blockdev-backup (state at the beginning) and blockdev-mirror > > (state at the end), except for a whole VM. >=20 > That doesn't seem practical unless you can instantaneously write out > the entire guest RAM to disk without blocking, or can somehow snapshot > the RAM so you can write out a consistent view of the original RAM, > while the guest continues to dirty RAM pages. People have suggested doing something like that with userfault write mode; but the same would also be doable just by write protecting the whole of RAM and then following the faults. Dave > Regards, > Daniel > --=20 > |: https://berrange.com -o- https://www.flickr.com/photos/dberr= ange :| > |: https://libvirt.org -o- https://fstop138.berrange= .com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberr= ange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK