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Subject: Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm
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To: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, ceph-devel@vger.kernel.org, Christian Brunner <chb@muc.de>

On 05/24/2010 02:42 PM, MORITA Kazutaka wrote:
>
>> 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.
>>
>>      
> Sorry, my explanation was unclear.  I'm not sure how running servers
> on localhost can solve the problem.
>    

The local server can convert from the local (nbd) protocol to the remote 
(sheepdog, ceph) protocol.

> What I wanted to say was that we cannot specify the image of VM. With
> nbd protocol, command line arguments are as follows:
>
>   $ qemu nbd:hostname:port
>
> As this syntax shows, with nbd protocol the client cannot pass the VM
> image name to the server.
>    

We would extend it to allow it to connect to a unix domain socket:

   qemu nbd:unix:/path/to/socket

The server at the other end would associate the socket with a filename 
and forward it to the server using the remote protocol.

However, I don't think nbd would be a good protocol.  My preference 
would be for a plugin API, or for a new local protocol that uses 
splice() to avoid copies.

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