[libvirt] How to avoid failure of migration/restoring/starting if cdrom is ejected inside guest?

[Osier Yang]

Hello list,

There is problem of migration if the changedable medium is ejected inside
guest, this is caused by qemu closes the block driver backend once the
medium is ejected, but it doesn't gives a way to let libvirt known the fact.
So, libvirt will try to migrate the guest with the media still exists.
This will
cause the failure, as qemu already closes the block driver backend.

Actually this could also break domain restoring and starting (if the domain
has a managed saving image, supposing the media is ejected before saving
or managed saving).

It's ideal if qemu could provide some event so that libvirt could known the
media is changed immediately, but it's bad news qemu upstream won't make
a patch for this in a short time.

As a alternative solution, they proposed patch to expose the status of
changeable medium via monitor command "info block":

http://lists.gnu.org/archive/html/qemu-devel/2011-08/msg00408.html

The output of the improved "info block" looks like below:

(qemu) info block
disk0: removable=0 file=/home/armbru/work/images/test.qcow2
backing_file=test.img ro=0 drv=qcow2 encrypted=0
cd: removable=1 locked=0 ejected file=x.iso ro=1 drv=raw encrypted=0

What libvirt can do with the qemu improvement is checking the meduim status
at the time of migration, but it doesn't kill all the bugs, such as for a
live migration, one can eject the media inside guest just during the migration
process. This probally makes a race and cause the failure just the same.

And moreover, this won't solve the problem on restoring and starting
with managed
saving image, can't get the medium status as the guest is not active.

So, I 'm hesitating to use "info block" to resolve the problems, it
can't resolve
the root problem thoroughly.

Or I missed some good ideas? Any thought is welcomed, thanks.

By the way, it might be deserving to report the cdrom tray status using
the improved
"info block" though, though qemu might keep improving the command to
output more
infomation, such as the media is inserted, but the tray is still open
(which means
different if we report the tray status as "close" if "info block"
outputs "inserted",
need to change the codes then)?

Patches of qemu side to improve the tray handling:

http://lists.nongnu.org/archive/html/qemu-devel/2011-06/msg00381.html

Regards
Osier