Re: [Qemu-devel] migration: new sections and backward compatibility.

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/07/2011 04:23 AM, Markus Armbruster wrote:
> Anthony Liguori<anthony@codemonkey.ws>  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?

Is that because 'scsi-cd' doesn't need a section or because it hasn't 
been implemented yet?

Regards,

Anthony Liguori

>