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Date: Mon, 24 May 2010 14:19:34 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm
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To: Avi Kivity <avi@redhat.com>
Cc: kvm@vger.kernel.org, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, ceph-devel@vger.kernel.org, Christian Brunner <chb@muc.de>

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<avi@redhat.com>  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