[Qemu-devel] Re: [RFC][PATCH] Option to continue after migration

[Jan Kiszka]

Nathan Baum wrote:
> I see from browsing the list a little more that I should have stuck
> [RFC] in my subject lin.
> 
> On Fri, 2009-09-18 at 02:52 +0100, Jamie Lokier wrote:
>> Nathan Baum wrote:
>>> Hello,
>>>
>>> This patch adds a -c "continue" option to the migrate command which
>>> causes the originating VM to remain running after migration, assuming it
>>> was running before.
>>>
>>> I've done this primarily so that I can do "migrate -d -c exec:cat>file"
>>> to take a snapshot without needing to have a snapshot-capable drive
>>> attached, or wait until migration is complete and continue the VM
>>> manually.
>> When you later resume from the snapshot, isn't the state of virtual
>> disks corrupted by the activity after the snapshot was taken?  When
>> the guest resumes, even though the disks will be corrupted, it won't
>> _know_ they are corrupted (unlike a reboot), so may proceed to make
>> things worse.
> 
> Yes. This is not specific to my patch, but it does limit its
> applicability.
> 
> You probably only want to snapshot-and-continue if you have no writable
> media -- i.e. you're booting a LiveCD, or you have writable media in
> snapshot mode, have stopped and committed immediately before saving the
> state this way, and haven't committed since.
> 
> In this latter case, my patch wouldn't be useful to you since you
> stopped the VM prior to the commit anyway, and my patch only continues
> if the VM is still running at the end of the migration. (Maybe it should
> force a continue; then you could chuck "stop\ncommit all\nmigrate -d
> -c ..." at the monitor.)
> 
> I think a far superior alternative approach would be to add "-snapshots
> file" option (or similar) to specify a file savevm/loadvm should use for
> VM state, and a "-snapshotsdir dir" option which makes QEMU quietly
> replace each not-snapshot-capable drive with a qcow2 based on it in that
> directory. (Or all the snapshot data could be in the snapshots file;
> that looks more complicated to implement.)

"Migration without shared storage" should once solve this, see Liran's
patches [1]. Your patch will be very helpful in that scenario.

Jan

[1] http://permalink.gmane.org/gmane.comp.emulators.qemu/52252

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