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

[Anthony Liguori]

On 05/24/2010 06:03 AM, Avi Kivity wrote:
> On 05/24/2010 11:27 AM, Stefan Hajnoczi wrote:
> > On Sun, May 23, 2010 at 1:01 PM, Avi Kivity<address@hidden>  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.
> > Besides a shared memory approach, I wonder if the splice() family of
> > syscalls could be used to send/receive data through a storage daemon
> > without the daemon looking at or copying the data?
>
> Excellent idea.

splice() eventually requires a copy.  You cannot splice() to linux-aio 
so you'd have to splice() to a temporary buffer and then call into 
linux-aio.  With shared memory, you can avoid ever bringing the data 
into memory via O_DIRECT and linux-aio.

Regards,

Anthony Liguori