Anthony Liguori <anth...@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?
