From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41159) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1elc8w-0004KP-8G for qemu-devel@nongnu.org; Tue, 13 Feb 2018 10:05:21 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1elc8u-0004lF-WB for qemu-devel@nongnu.org; Tue, 13 Feb 2018 10:05:14 -0500 Date: Tue, 13 Feb 2018 15:05:03 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20180213150503.GF2378@work-vm> References: <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> <20180213145913.GE2378@work-vm> <6845a694-aa22-90e8-7a9b-ce0283be450c@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <6845a694-aa22-90e8-7a9b-ce0283be450c@virtuozzo.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: "Denis V. Lunev" Cc: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , 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 , aarcange@redhat.com * Denis V. Lunev (den@virtuozzo.com) wrote: > On 02/13/2018 05:59 PM, Dr. David Alan Gilbert wrote: > > * 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 lo= ading > >>>>>>> and saving snapshots. > >>>>>> Yes, you could. I think for a proper implementation you would wa= nt 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, yo= u 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. > >>>>> > >>>>> 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. > >>>> No, that's policy decision that doesn't matter from QMP pov. If th= e mgmt > >>>> app wants the snapshot to be wrt to the initial time, it can simpl= y > >>>> invoke the "stop" QMP command before doing the live migration and > >>>> "cont" afterwards. > >>> That would be non-live. I think Roman means a live snapshot that sa= ves > >>> the state at the beginning of the operation. Basically the differen= ce > >>> between blockdev-backup (state at the beginning) and blockdev-mirro= r > >>> (state at the end), except for a whole VM. > >> That doesn't seem practical unless you can instantaneously write out > >> the entire guest RAM to disk without blocking, or can somehow snapsh= ot > >> 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. >=20 > nope, userfault fd does not help :( We have tried, the functionality is= not > enough. Better to have small extension to KVM to protect all memory > and notify QEMU with accessed address. Can you explain why? I thought the write-protect mode of userfaultfd was supposed to be able to do that; cc'ing in Andrea Dave > Den -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK