Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm

[MORITA Kazutaka]

At Sun, 23 May 2010 15:01:59 +0300,
Avi Kivity wrote:
> 
> On 05/21/2010 12:29 AM, Anthony Liguori wrote:
> >
> > I'd be more interested in enabling people to build these types of 
> > storage systems without touching qemu.
> >
> > Both sheepdog and ceph ultimately transmit I/O over a socket to a 
> > central daemon, right? 
> 
> That incurs an extra copy.
> 
> > So could we not standardize a protocol for this that both sheepdog and 
> > ceph could implement?
> 
> The protocol already exists, nbd.  It doesn't support snapshotting etc. 
> but we could extend it.
> 

I have no objection to use another protocol for Sheepdog support, but
I think nbd protocol is unsuitable for the large storage pool with
many VM images.  It is because nbd protocol doesn't support specifing
a file name to open.  If we use nbd with such a storage system, the
server needs to listen ports as many as the number of VM images.  As
far as I see the protocol, It looks difficult to extend it without
breaking backward compatibility.

Regards,

Kazutaka

> But IMO what's needed is a plugin API for the block layer.
>