Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

[Anthony Liguori]

On 03/02/2011 08:00 AM, Avi Kivity wrote:
> On 03/02/2011 02:39 PM, Anthony Liguori wrote:
> > Here is where your race is:
> >
> > 2. Management sends a switch command
> >
> > 3. QEMU receives switch command
> >
> > 4. QEMU stops doubling IO and switches to the destination
> >
> > 5. QEMU sends acknowledgement of switch command
> >
> > 6. Management receives acknowledge of switch command
> >
> > 7. Management changes internal state definition to reflect the new 
> > destination
> >
> > If QEMU or the management tool crashes after step 4 and before step 
> > 6, when the management tool restarts QEMU with the source image, data 
> > loss will have occurred (and potentially corruption if a flush had 
> > happened).
>
> No.  After step 2, any qemu restart will be with the destination 
> image.  If the management tool restarts, it can query the state (or 
> just re-issue the switch command, which is idempotent).

Yeah, I think you're right, although I need to think through it a bit more.

Regards,

Anthony Liguori