Re: [Qemu-devel] [Qemu-block] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Kevin Wolf]

Am 13.02.2018 um 12:51 hat Daniel P. Berrangé geschrieben:
> On Tue, Feb 13, 2018 at 11:43:55AM +0000, Dr. David Alan Gilbert wrote:
> > * Kevin Wolf (address@hidden) 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:<name>
> > > > 
> > > > where '<name>' 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=<node-name>,name=<snapshot-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.
> 
> The commands block the monitor too, so I always assumed they were non-live

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