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

[Kevin Wolf]

Am 13.02.2018 um 15:58 hat Daniel P. Berrangé geschrieben:
> On Tue, Feb 13, 2018 at 03:43:10PM +0100, Kevin Wolf wrote:
> > Am 13.02.2018 um 15:30 hat Roman Kagan geschrieben:
> > > 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 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 [...]
> > > 
> > > 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.
> > 
> > I don't think it's necessarily a big problem as long as you set the
> > expectations right, but good point anyway.
> > 
> > > 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.
> > 
> > So basically you would let the guest fault whenever it writes to a page
> > that is not saved yet, and then save it first before you make the page
> > writable again? Essentially blockdev-backup, except for RAM.
> 
> The page fault servicing will be delayed by however long it takes to
> write the page to underling storage, which could be considerable with
> non-SSD. So guest performance could be significantly impacted on slow
> storage with high dirtying rate. On the flip side it gurantees a live
> snapshot would complete in finite time which is good.

You can just use a bounce buffer for writing out the old page. Then the
VM is only stopped for the duration of a malloc() + memcpy().

Kevin