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Re: [Qemu-devel] migration: new sections and backward compatibility.
From: |
Alexander Graf |
Subject: |
Re: [Qemu-devel] migration: new sections and backward compatibility. |
Date: |
Thu, 7 Jul 2011 13:02:11 +0200 |
On 07.07.2011, at 11:23, Markus Armbruster wrote:
> Anthony Liguori <address@hidden> writes:
>
>> On 07/06/2011 11:04 AM, Gerd Hoffmann wrote:
>>> Hi folks,
>>>
>>> We'll need to figure a sane way to handle migration to older versions
>>> with new sections, i.e. devices which used to not save state before do now.
>>>
>>> We already have one case in tree: usb. qemu 0.14 saves state for usb-hid
>>> devices and the usb-hub, whereas qemu 0.13 and older don't. You can't
>>> migrate a vm with a usb-tablet from 0.14 to 0.13 because of that even if
>>> you use -M pc-0.13.
>>
>> Because if you did migrate, you would actively break the guest during
>> migration. So why is this a problem?
>>
>> This comes up a lot. We shouldn't enable migration if we know the
>> guest is going to break during migration. That's a feature, not a
>> bug.
>
> Not so fast :)
>
> I agree that throwing away unrecognized migration data is unsafe, and
> should not be done. Now let me present my little problem.
>
> I'm working on making migration preserve "tray status": open/closed,
> locked/unlocked.
>
> For ide-cd, I can stick a subsection "ide_drive/tray_state" into section
> "ide_drive". Needed only if the tray is open or locked. This gives
> users a chance to migrate to older versions, and is perfectly safe.
>
> scsi-cd doesn't have a section, yet. What now?
I guess the obvious answer would be "There shouldn't be devices that don't have
a section" :). Not sure how to not break old -M with this though :(. I'd guess
the best would be to have a special VMSTATE that means "broken old version
doesn't send a section" which we can set for special -M?
That would of course not help with RHEL :).
Alex
Re: [Qemu-devel] migration: new sections and backward compatibility., Avi Kivity, 2011/07/06