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

[Anthony Liguori]

On 05/24/2010 06:56 AM, Avi Kivity wrote:
> 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

nbd is a no-go because it only supports a single, synchronous I/O 
operation at a time and has no mechanism for extensibility.

If we go this route, I think two options are worth considering.  The 
first would be a purely socket based approach where we just accepted the 
extra copy.

The other potential approach would be shared memory based.  We export 
all guest ram as shared memory along with a small bounce buffer pool.  
We would then use a ring queue (potentially even using virtio-blk) and 
an eventfd for notification.

> 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.

I think a good shared memory implementation would be preferable to 
plugins.  I think it's worth attempting to do a plugin interface for the 
block layer but I strongly suspect it would not be sufficient.

I would not want to see plugins that interacted with BlockDriverState 
directly, for instance.  We change it far too often.  Our main loop 
functions are also not terribly stable so I'm not sure how we would 
handle that (unless we forced all block plugins to be in a separate thread).

Regards,

Anthony Liguori