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

[Markus Armbruster]

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?