Using loadvm with snapshot

Using loadvm with snapshot

[Gabriel Southern]

Hi,

Are there plans to support using -loadvm with -snapshot?

I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:

https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/

Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.

I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.

Thanks,

-Gabriel

Re: Using loadvm with snapshot

[Dr. David Alan Gilbert]

* Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> Hi,
> 
> Are there plans to support using -loadvm with -snapshot?
> 
> I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> 
> https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> 
> Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> 
> I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.

One thing that might work for you is migrate to a file;

 (qemu)  migrate_set_parameter max-bandwidth 100G
 (qemu) migrate "exec:cat > myfile.mig"
 (qemu) q


qemu -incoming "exec:cat myfile.mig"

Dave
> 
> Thanks,
> 
> -Gabriel
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: Using loadvm with snapshot

[Peter Maydell]

On Tue, 17 Aug 2021 at 17:27, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
>
> * Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> > Hi,
> >
> > Are there plans to support using -loadvm with -snapshot?
> >
> > I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> >
> > https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> >
> > Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> >
> > I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.

Do you know why -loadvm and -snapshot don't work together?
It doesn't on the face of it seem like they would be incompatible...

-- PMM

Re: Using loadvm with snapshot

[Dr. David Alan Gilbert]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Tue, 17 Aug 2021 at 17:27, Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> >
> > * Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> > > Hi,
> > >
> > > Are there plans to support using -loadvm with -snapshot?
> > >
> > > I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> > >
> > > https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> > >
> > > Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> > >
> > > I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.
> 
> Do you know why -loadvm and -snapshot don't work together?
> It doesn't on the face of it seem like they would be incompatible...

No I don't; I've never actually used -loadvm - given that both the
snapshot mechanism and the load/savevm mechanism are fairly special
qcow2 hacks, it'll be a qcow2 level thing.

Dave

> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: Using loadvm with snapshot

[Kevin Wolf]

Am 17.08.2021 um 20:48 hat Peter Maydell geschrieben:
> On Tue, 17 Aug 2021 at 17:27, Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> >
> > * Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> > > Hi,
> > >
> > > Are there plans to support using -loadvm with -snapshot?
> > >
> > > I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> > >
> > > https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> > >
> > > Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> > >
> > > I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.
> 
> Do you know why -loadvm and -snapshot don't work together?
> It doesn't on the face of it seem like they would be incompatible...

Probably because -snapshot first adds a temporary qcow2 overlay to the
image, and then -loadvm fails because the newly created temporary
overlay doesn't contain the requested snapshot.

Internal and external snapshots don't really mix well.

Kevin

Re: Using loadvm with snapshot

[Peter Maydell]

On Tue, 24 Aug 2021 at 16:21, Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 17.08.2021 um 20:48 hat Peter Maydell geschrieben:
> > On Tue, 17 Aug 2021 at 17:27, Dr. David Alan Gilbert
> > <dgilbert@redhat.com> wrote:
> > >
> > > * Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> > > > Hi,
> > > >
> > > > Are there plans to support using -loadvm with -snapshot?
> > > >
> > > > I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> > > >
> > > > https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> > > >
> > > > Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> > > >
> > > > I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.
> >
> > Do you know why -loadvm and -snapshot don't work together?
> > It doesn't on the face of it seem like they would be incompatible...
>
> Probably because -snapshot first adds a temporary qcow2 overlay to the
> image, and then -loadvm fails because the newly created temporary
> overlay doesn't contain the requested snapshot.

That sounds like the bug could be fixed by having "find the
snapshot" look not just in the overlay but also down into
the underlying image?

-- PMM

Re: Using loadvm with snapshot

[Kevin Wolf]

Am 24.08.2021 um 17:25 hat Peter Maydell geschrieben:
> On Tue, 24 Aug 2021 at 16:21, Kevin Wolf <kwolf@redhat.com> wrote:
> >
> > Am 17.08.2021 um 20:48 hat Peter Maydell geschrieben:
> > > On Tue, 17 Aug 2021 at 17:27, Dr. David Alan Gilbert
> > > <dgilbert@redhat.com> wrote:
> > > >
> > > > * Gabriel Southern (gsouther@qti.qualcomm.com) wrote:
> > > > > Hi,
> > > > >
> > > > > Are there plans to support using -loadvm with -snapshot?
> > > > >
> > > > > I saw some past discussion on mailing list including bug that was closed earlier this year but no recent activity:
> > > > >
> > > > > https://lore.kernel.org/qemu-devel/162424905685.11837.7303570061857383641.malone@loganberry.canonical.com/
> > > > >
> > > > > Testing with latest qemu it looks like -loadvm still does not work when combined with -snapshot.
> > > > >
> > > > > I'm curious how complex it would be to implement this feature and if it may show up on QEMU roadmap in the future. Or if there is alterative command that can be used to save the state of a running VM and then use the same qcow to run multiple QEMU instances loading this VM state running in snapshot mode.
> > >
> > > Do you know why -loadvm and -snapshot don't work together?
> > > It doesn't on the face of it seem like they would be incompatible...
> >
> > Probably because -snapshot first adds a temporary qcow2 overlay to the
> > image, and then -loadvm fails because the newly created temporary
> > overlay doesn't contain the requested snapshot.
> 
> That sounds like the bug could be fixed by having "find the
> snapshot" look not just in the overlay but also down into
> the underlying image?

In this specific case yes, because we know that the overlay was just
created and is still empty.

In the general case, loading internal snapshots on a backing file would
be wrong because the data in the overlay would be from a different time
than the data in the backing file, probably causing data corruption.

Kevin