From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:32958) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1elZ0c-0003bh-TT for qemu-devel@nongnu.org; Tue, 13 Feb 2018 06:44:28 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1elZ0c-0004Qn-1i for qemu-devel@nongnu.org; Tue, 13 Feb 2018 06:44:26 -0500 Date: Tue, 13 Feb 2018 11:43:55 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20180213114355.GC2378@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180213105024.GC5083@localhost.localdomain> 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: Kevin Wolf Cc: "Daniel P. Berrange" , Max Reitz , Richard Palethorpe , Qemu-block , quintela@redhat.com, armbru@redhat.com, qemu-devel@nongnu.org, rpalethorpe@suse.com * 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 functionality. These > > > >> functions behave the same way as the HMP savevm, loadvm and delvm > > > >> commands. This will allow applications, such as OpenQA, to programmatically > > > >> revert the VM to a previous state with no dependence on HMP or qemu-img. > > > > > > > > That's already possible; libvirt uses QMP's human-monitor-command to > > > > access these HMP commands programmatically. > > > > > > > > We've had discussions in the past about what it would take to have > > > > specific QMP commands for these operations; the biggest problem is that > > > > these commands promote the use of internal snapshots, and there are > > > > enough performance and other issues with internal snapshots that we are > > > > not yet ready to commit to a long-term interface for making their use > > > > easier. At this point, our recommendation is to prefer external snapshots. > > > > > > We already have QMP commands for internal snapshots, though. Isn't the > > > biggest issue that savevm takes too much time to be a synchronous QMP > > > command? > > > > Ultimately savevm/loadvm are using much of the migration code internally, > > but are not exposed as URI schemes. Could we perhaps take advantage of > > the internal common layer and define a migration URI scheme > > > > snapshot: > > > > where '' is the name of the internal snapshot in the qcow2 file. > > Let's include a node-name there, please. QEMU automagically deciding > where to store the VM state is one of the major problems of the HMP > interface. > > And while we're at it, we can make it more future-proof by allowing to > specify arbitrary options: > > snapshot:node=,name= > > That would allow us to add something like compressed=on|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. > > > 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, you don't want to > waste space by keeping both the old and the current version of the page > content in the file. The current snapshots are run with the CPU paused aren't they? They share exactly the same RAM saving code as migration. Dave > Kevin -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK