[PATCH 1/3] qemu: implementation of transient option for qcow2 file

[Masayoshi Mizuma]

On Sat, Jul 18, 2020 at 08:06:00AM +0200, Peter Krempa wrote:
> On Thu, Jul 16, 2020 at 20:55:29 -0400, Masayoshi Mizuma wrote:
> > Thank you for your review.
> > 
> > On Tue, Jul 07, 2020 at 06:36:23AM -0500, Eric Blake wrote:
> > > On 7/7/20 2:12 AM, Peter Krempa wrote:
> > > 
> > > > You can install a qcow2 overlay on top of a raw file too. IMO the
> > > > implications of using <transient/> allow that.
> > > > 
> > > > As said above I'd strongly prefer if the overlay is created in qemu
> > > > using the blockdev-create blockjob (there is already infrastructure in
> > > > libvirt to achieve that).
> > > 
> > > Agreed.  At this point, any time we call out to qemu-img as a separate
> > > process, we are probably doing it wrong.
> > 
> > Got it. I'm thinking about the procedure such as followings.
> > Does that make sense?
> > 
> >   1) Open the monitor with qemuProcessQMPNew()/qemuProcessQMPStart(), 
> >      and connect it.
> 
> Starting a new qemu process just to format an image is extreme overkill
> and definitely not what we want to do.

I see, thanks.

> 
> >   2) Setup the transient disk with qemuDomainPrepareStorageSourceBlockdev(),
> >      qemuBlockStorageSourceAttachApplyStorage(), qemuBlockStorageSourceCreateGetFormatProps()
> >      and something...
> > 
> >   3) Run blockdev-create command with qemuMonitorBlockdevCreate(), then
> >      close the monitor.
> 
> These two steps should be exectued in the qemu process which already
> will run the VM prior to starting the guest CPUs.
> 
> >   4) Switch the original disk to the transient disk.
> > 
> >   5) Build the blockdev argument for qemu.
> 
> And instead of this step, you use the external snapshot infrastructure
> to install the overlays via 'blockdev-snapshot' QMP command

OK. I suppose qemuDomainSnapshotDiskPrepare() and
qemuDomainSnapshotDiskUpdateSource() maybe helpful to implement the
setup steps of transient disk.

> 
> > 
> >   6) Run qemu
> 
> And instead of this the VM cpus will be started.

Got it, I think the appropriate place is after virCommandRun() at qemuProcessLaunch(),
and before qemuProcessFinishStartup().

> 
> 
> The above steps require factoring out snapshot code a bit. I have a few
> patches in that direction so I'll try posting them next week hopefully.

Great, thanks!

- Masa