From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40978) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1elaVV-0005mZ-LW for qemu-devel@nongnu.org; Tue, 13 Feb 2018 08:20:26 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1elaVU-0002Qv-Be for qemu-devel@nongnu.org; Tue, 13 Feb 2018 08:20:25 -0500 Date: Tue, 13 Feb 2018 14:20:00 +0100 From: Kevin Wolf Message-ID: <20180213132000.GG5083@localhost.localdomain> References: <20180107122336.29333-1-richiejp@f-m.fm> <5cf19623-72ac-fb8b-2054-a60d42419ec6@redhat.com> <20180111130427.GG8326@redhat.com> <20180213105024.GC5083@localhost.localdomain> <20180213114355.GC2378@work-vm> <20180213115102.GF573@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20180213115102.GF573@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: "Dr. David Alan Gilbert" , Max Reitz , Richard Palethorpe , Qemu-block , quintela@redhat.com, armbru@redhat.com, qemu-devel@nongnu.org, rpalethorpe@suse.com Am 13.02.2018 um 12:51 hat Daniel P. Berrang=E9 geschrieben: > On Tue, Feb 13, 2018 at 11:43:55AM +0000, Dr. David Alan Gilbert wrote: > > * Kevin Wolf (kwolf@redhat.com) wrote: > > > Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben: > > > > On Thu, Jan 11, 2018 at 01:46:38PM +0100, Max Reitz wrote: > > > > > On 2018-01-08 14:52, Eric Blake wrote: > > > > > > On 01/07/2018 06:23 AM, Richard Palethorpe wrote: > > > > > >> Add QAPI wrapper functions for the existing snapshot functio= nality. These > > > > > >> functions behave the same way as the HMP savevm, loadvm and = delvm > > > > > >> commands. This will allow applications, such as OpenQA, to p= rogrammatically > > > > > >> revert the VM to a previous state with no dependence on HMP = or qemu-img. > > > > > >=20 > > > > > > That's already possible; libvirt uses QMP's human-monitor-com= mand to > > > > > > access these HMP commands programmatically. > > > > > >=20 > > > > > > We've had discussions in the past about what it would take to= have > > > > > > specific QMP commands for these operations; the biggest probl= em is that > > > > > > these commands promote the use of internal snapshots, and the= re are > > > > > > enough performance and other issues with internal snapshots t= hat we are > > > > > > not yet ready to commit to a long-term interface for making t= heir use > > > > > > easier. At this point, our recommendation is to prefer exter= nal snapshots. > > > > >=20 > > > > > We already have QMP commands for internal snapshots, though. I= sn't the > > > > > biggest issue that savevm takes too much time to be a synchrono= us QMP > > > > > command? > > > >=20 > > > > Ultimately savevm/loadvm are using much of the migration code int= ernally, > > > > but are not exposed as URI schemes. Could we perhaps take advanta= ge of > > > > the internal common layer and define a migration URI scheme > > > >=20 > > > > snapshot: > > > >=20 > > > > where '' is the name of the internal snapshot in the qcow2 = file. > > >=20 > > > Let's include a node-name there, please. QEMU automagically decidin= g > > > where to store the VM state is one of the major problems of the HMP > > > interface. > > >=20 > > > And while we're at it, we can make it more future-proof by allowing= to > > > specify arbitrary options: > > >=20 > > > snapshot:node=3D,name=3D > > >=20 > > > That would allow us to add something like compressed=3Don|off later= . > > > Actually, compressed VM state sounds pretty nice. Why don't we have= this > > > yet? The qcow2 format already provides everything you need for it. > > >=20 > > > > Then you could just use the regular migrate QMP commands for load= ing > > > > and saving snapshots. > > >=20 > > > 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 n= ot > > > really well suited for snapshots. When a RAM page is dirtied, you j= ust > > > want to overwrite the old version of it in a snapshot, you don't wa= nt to > > > waste space by keeping both the old and the current version of the = page > > > content in the file. > >=20 > > The current snapshots are run with the CPU paused aren't they? They > > share exactly the same RAM saving code as migration. >=20 > The commands block the monitor too, so I always assumed they were non-l= ive Yes, they are non-live, so RAM simply doesn't change and we that's the reason why we currently don't get duplicate pages in the snapshot. But as I understand it, the whole point of migrate snapshot:... would be to make it live, so the potential duplication is something that we need to consider for it. Kevin