Re: [Qemu-devel] [PATCH v2 1/1] Add QMP bits for blockdev-snapshot-sync.

[Anthony Liguori]

On 04/28/2011 09:38 AM, Jes Sorensen wrote:
> Sorry but this is utterly bogus.
>
> The snapshot support as is works fine, and the command is in the
> monitor. We should expose it in QMP as well.

It went in for the monitor because it was considered an imperfect 
command so we held up the QMP side because we wanted a better interface.

The current command does:

1) Create new image backing to current image

2) Flush outstanding I/O to old image

3) Close current image

4) Reopen newly created image

5) Go

Operations (1) and (2) are very synchronous operations.  (4) can be too. 
 We really should have a bdrv_aio_snapshot() function that implements 
the logic for at least (2) in an asynchronous fashion.

That sort of interface is going to affect how we expose things in QMP. 
As from a QMP perspective, we're going to do something like:

a) start snapshot

b) query snapshot progress

c) receive notification of snapshot completion

d) flip over image

And of course, this needs to be carefully thought through for race 
conditions.  In the current command, what happens if you get a crash 
between (2) and (3)?  There's no way for the management tools to know 
that we didn't finish flushing writes.  How does the management tool 
know that (1) didn't fail mid way through resulting in a corrupted image?

Regards,

Anthony Liguori