Re: [Qemu-devel] [PATCH V2 3/4] blockdev: Allow snapshoting of protocols.

[Kevin Wolf]

Am 29.01.2013 14:45, schrieb Benoît Canet:
> Le Tuesday 29 Jan 2013 à 14:25:42 (+0100), Kevin Wolf a écrit :
>> Am 29.01.2013 14:07, schrieb Benoît Canet:
>>>> Wait, what's happening here? I don't understand this patch and how it's
>>>> related to snapshotting non-file protocols (if this is even what you
>>>> mean). What is your exact scenario, what does the existing code do in
>>>> it, and how does this change improve it? An empty commit message is
>>>> definitely not appropriate for such a change.
>>>>
>>>> In any case, using NULL as drv for bdrv_open() looks plain wrong.
>>>
>>> When passing drv bdrv_open tries to open the url as qcow2 or another plain 
>>> file
>>> format and fail.
>>
>> What exactly is a "plain file format" and what is the difference between
>> it and other file formats?
> 
> Quorum is a protocol which encapsulate n BlockDriverState.

So it's not a file format, neither plain nor otherwise.

>> Anyway, this is not a problem description, let alone a description of
>> your exact scenario.
> 
> A scenario would be:
> A user want to create a new quorum snapshot made of n qcow2 snapshots.
> The user specify qcow2 as format so the new quorum files are created as qcow2.
> Then the code try to open the quorum file as a qcow2 file and it fail.

Like this?

base1 [file] --- base1 [qcow2] --\
base2 [file] --- base2 [qcow2] --->-- quorum --- snap.qcow2
base3 [file] --- base3 [qcow2] --/

Or like this? (This is the only one where quorum is really used as  a
protocol)

base1 [file] --\
base2 [file] --->-- quorum --- base [qcow2] --- snap.qcow2
base3 [file] --/

Or even this one?

base1 [file] --- base1 [qcow2] --- snap1.qcow2 --\
base2 [file] --- base2 [qcow2] --- snap2.qcow2 --->-- quorum
base3 [file] --- base3 [qcow2] --- snap3.qcow2 --/

I think the last one is what you really want, but it's certainly not the
case that is enabled by this patch.

>> If qcow2 is specified as the format in the QMP command, why is it then
>> wrong to try and open the file as qcow2? Why does it fail? What is the
>> correct driver?
> 
> It's correct to format the individual quorum files as qcow2 so specifying 
> qcow2
> in the qmp command seems ok.
> But opening a "quorum:" url which agregate these qcow2 files will fail if drv 
> is
> the qcow2 driver.
> Would passing the quorum protocol driver to bdrv_open be ok ?

A quorum: URL opened with the qcow2 driver should succeed and result in
something like the second case mentioned above.

Kevin