Re: [Qemu-devel] [RFC PATCH v3 06/11] qemu-ga: Add Windows VSS requester to quisce applications and filesystems

[Tomoki Sekiyama]

On 5/21/13 12:56 , "Eric Blake" <address@hidden> wrote:

>On 05/21/2013 09:33 AM, Tomoki Sekiyama wrote:
>> Add VSS requester functions for to qemu-ga.
>> This provides facility to request VSS service in Windows guest to quisce
>
>s/quisce/quiesce/

Oops, thanks again.

>> applications and filesystems. This function is only supported in Windows
>> 2003 or later. In older guests, this function does nothing.
>> 
>> In several versions of Windows which don't support attribute
>> VSS_VOLSNAP_ATTR_NO_AUTORECOVERY, DoSnapshotSet fails with error
>> VSS_E_OBJECT_NOT_FOUND. In this patch, we just ignore this error.
>> To solve this fundamentally, we need a framework to handle mount
>>writable
>> snapshot on guests, which is required by VSS auto-recovery feature
>> (a cleanup phase after snapshot is taken).
>
>I take it that such a framework may involve additional guest-agent
>commands and management work (such as libvirt having to be aware of the
>added framework).  How important is this functionality?  How frequently
>does VSS even get us into that state?  That is, are Windows guests going
>to frequently hit our current first-round implementation limits that
>fail to let them do auto-recovery?

"Auto-recovery" is invoked every time after snapshots are taken by VSS
(the name is misleading, but it is normal path...) to make the snapshot
completely consistent state by rolling back incomplete transactions.
VSS requesters can explicitly disable this, but this may remain some
incomplete data in the snapshot when some applications which relying
on this feature are running. However, I believe they can usually be
recovered *after the snapshot is rolled back*.

Full-support of VSS including auto-recovery may require the other channel
between libvirt and guest-agent to conduct snapshot operation from guests.
I think such interface can also enable various VSS feature such as partial-
recovery from snapshots. It would be nice to have in the future version,
but is overkill just to enable fsfreeze command so far....


Thanks,
Tomoki Sekiyama