Re: [Qemu-devel] Signal handling and qcow2 image corruption

[David Barrett <dbarrett@quinthar.com>]

Ah thanks, that makes a lot of sense.  Unfortunately, -snapshot doesn't 
appear to work with -loadvm: any VM snapshots created outside of 
snapshot mode are suppressed, and any VM snapshots created inside 
snapshot mode disappear on close (even if you try to commit):

===== I have a snapshot VM named 'boot' that -snapshot can't find ====
dbarrett@LappyReborn:~/rs/qa$ qemu -smb qemu -kernel-kqemu -localtime -m 
512 -monitor stdio -loadvm boot -snapshot winxp.qcow2
QEMU 0.9.0 monitor - type 'help' for more information
(qemu) Could not find snapshot 'boot' on device 'hda'
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID        TAG                 VM SIZE                DATE       VM CLOCK

============ When I try to save one it appears to work.... ===========
(qemu) savevm boot
(qemu) commit all
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         boot                    27M 2008-03-05 15:37:35   00:00:23.114
(qemu) quit

==== ...but when I start up again with -snapshot, it can't find it ====
dbarrett@LappyReborn:~/rs/qa$ qemu -smb qemu -kernel-kqemu -localtime -m 
512 -monitor stdio -loadvm boot -snapshot winxp.qcow2
QEMU 0.9.0 monitor - type 'help' for more information
(qemu) Could not find snapshot 'boot' on device 'hda'
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID        TAG                 VM SIZE                DATE       VM CLOCK
(qemu)


=== But when -snapshot is disabled, it finds my snapshot VM again ====
dbarrett@LappyReborn:~/rs/qa$ qemu -smb qemu -kernel-kqemu -localtime -m 
512 -monitor stdio -loadvm boot winxp.qcow2
QEMU 0.9.0 monitor - type 'help' for more information
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         boot                    53M 2008-03-03 17:30:58   01:40:10.163
(qemu)


I think the solution is to skip -snapshot, use -loadvm, and just use a 
named pipe to send a "quit" command to the monitor in order to shut it 
down rather than SIGTERM.

Thanks for your help!

-david

Anthony Liguori wrote:
> David Barrett wrote:
>> I'm tracking down a image corruption issue and I'm curious if you can 
>> answer the following:
>>
>> 1) Is there any difference between sending a "TERM" signal to the QEMU 
>> process and typing "quit" at the monitor?
> 
> Yes.  Since QEMU is single threaded, when you issue a quit, you know you 
> aren't in the middle of writing qcow2 meta data to disk.
> 
>> 2) Will sending TERM corrupt the 'gcow2' image (in ways other than 
>> normal guest OS dirty shutdown)?
> 
> Possibly, yes.
> 
>> 3) Assuming I always start QEMU using "-loadvm", is there any risk in 
>> using 'kill' to send SIGTERM to the QMEU process when done?
> 
> Yes.  If you want to SIGTERM QEMU, the safest thing to do is use -snapshot.
> 
> Regards,
> 
> Anthony Liguori
> 
> 
> 
>