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

[Avi Kivity <avi@redhat.com>]

On 05/24/2010 10:12 AM, MORITA Kazutaka wrote:
> 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.
>    

The server would be local and talk over a unix domain socket, perhaps 
anonymous.

nbd has other issues though, such as requiring a copy and no support for 
metadata operations such as snapshot and file size extension.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.