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

[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.

> 
> 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.

> 
> 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 ?

Benoît

> 
> > bdrv_file_open is not a better option because it won't return a 
> > BlockDriverState
> > constructed in the same way as the old one.
> > (raw as bs and quorum as ->file)
> > 
> > I agree that this patch is a hack and I am looking for a better way of 
> > doing it.
> 
> Let's talk about the problem first before discussing solutions.
> 
> Kevin