Re: [PATCH 1/3] qemu: implementation of transient option for qcow2 file
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.
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
6) Run qemu
And instead of this the VM cpus will be started.
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.