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Date: Wed, 02 Feb 2011 09:48:26 +0100
From: Jes Sorensen <Jes.Sorensen@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] Add virtagent file system freeze/thaw
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: agl@us.ibm.com, qemu-devel@nongnu.org, mdroth@linux.vnet.ibm.com

On 02/02/11 08:57, Stefan Hajnoczi wrote:
> On Tue, Feb 1, 2011 at 10:58 AM,  <Jes.Sorensen@redhat.com> wrote:
>> From: Jes Sorensen <Jes.Sorensen@redhat.com>
>>
>> Implement freeze/thaw support in the guest, allowing the host to
>> request the guest freezes all it's file systems before a live snapshot
>> is performed.
>>  - fsfreeze(): Walk the list of mounted local real file systems,
>>               and freeze them.
> 
> Does this add a requirement that guest agent code issues no disk I/O
> in its main loop (e.g. logging)?  Otherwise we might deadlock
> ourselves waiting for I/O which is never issued.

Yes very much so[1] - one reason why it would be nice to have virtagent
use threads to execute the actual commands. We should probably add a
flag to agent commands indicating whether they issue disk I/O or not, so
we can block attempts to execute commands that do so, while the guest is
frozen.

Cheers,
Jes

[1] speaking from experience ... a Linux desktop gets really upset if
you freeze the file systems from a command in an xterm.... ho hum