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

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm
Date: Mon, 24 May 2010 14:05:58 +0300
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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 

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