From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40276) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1elc5K-0000Q9-5I for qemu-devel@nongnu.org; Tue, 13 Feb 2018 10:01:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1elc5E-0001oK-FX for qemu-devel@nongnu.org; Tue, 13 Feb 2018 10:01:30 -0500 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> <20180213145913.GE2378@work-vm> From: "Denis V. Lunev" Message-ID: <6845a694-aa22-90e8-7a9b-ce0283be450c@virtuozzo.com> Date: Tue, 13 Feb 2018 18:01:17 +0300 MIME-Version: 1.0 In-Reply-To: <20180213145913.GE2378@work-vm> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US 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: "Dr. David Alan Gilbert" , "=?UTF-8?Q?Daniel_P._Berrang=c3=a9?=" 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 On 02/13/2018 05:59 PM, Dr. David Alan Gilbert wrote: > * Daniel P. Berrang=C3=A9 (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=C3=A9 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 loadin= g >>>>>>> and saving snapshots. >>>>>> Yes, you could. I think for a proper implementation you would want t= o do >>>>>> better, though. Live migration provides just a stream, but that's no= t >>>>>> really well suited for snapshots. When a RAM page is dirtied, you ju= st >>>>>> 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 late= r. >>>>> >>>>> I'm not sure this is what a user expects. >>>>> >>>>> 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. >>>>> No multiple copies of the same page would have to be saved so the str= eam >>>>> format would be fine. For the load part the usual inmigrate should >>>>> work. >>>> No, that's policy decision that doesn't matter from QMP pov. If the mg= mt >>>> 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. >>> That would be non-live. I think Roman means a live snapshot that saves >>> 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. >> 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. 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. Den