Re: [Qemu-devel] [RFC PATCH 0/2] Sheepdog: distributed storage system for QEMU

[MORITA Kazutaka]

At Thu, 13 May 2010 04:46:46 +0900,
MORITA Kazutaka wrote:
> 
> On 2010/05/12 20:38, Kevin Wolf wrote:
> > I'll have a closer look at your code later, but one thing I noticed is
> > that the new block driver is something in between a protocol and a
> > format driver (just like vvfat, which should stop doing so, too). I
> > think it ought to be a real protocol with the raw format driver on top
> > (or any other format - I don't see a reason why this should be
> > restricted to raw).
> > 
> > The one thing that is unusual about it as a protocol driver is that it
> > supports snapshots. However, while it is the first one, supporting
> > snapshots in protocols is a thing that could be generally useful to
> > support (for example thinking of a LVM protocol, which was discussed in
> > the past).
> > 
> 
> I agreed.  I'll modify the sheepdog driver patch as a protocol driver one,
> and remove unnecessary format check from my patch.
> 
> > So in block.c we could check if the format driver supports snapshots,
> > and if it doesn't we try again with the underlying protocol. Not sure
> > yet what we would do when both format and protocol do support snapshots
> > (qcow2 on sheepdog/LVM/...), but that's a detail.
> > 
> 

To support snapshot in a protocol, I'd like to call the hander of the
protocol driver in the following functions in block.c:

    bdrv_snapshot_create
    bdrv_snapshot_goto
    bdrv_snapshot_delete
    bdrv_snapshot_list
    bdrv_save_vmstate
    bdrv_load_vmstate

Is it okay?

In the case both format and protocol drivers support snapshots, I
think it is better to call the format driver handler.  Because qcow2
is well known as a snapshot support format, so when users use qcow2,
they expect to get snapshot with qcow2.

There is another problem to make the sheepdog driver be a protocol;
how to deal with protocol specific create_options?

For example, sheepdog supports cloning images as a format driver:

  $ qemu-img create -f sheepdog dst -b sheepdog:src

But if the sheepdog driver is a protocol, error will occur.

  $ qemu-img create sheepdog:dst -b sheepdog:src
  Unknown option 'backing_file'
  qemu-img: Backing file not supported for file format 'raw'

It is because the raw format doesn't support a backing_file option.
To support the protocol specific create_options, if the format driver
cannot parse some of the arguments, the protocol driver need to parse
them.

If my suggestions are okay, I'd like to prepare the patches.

Regards,

Kazutaka