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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: Tue, 25 May 2010 12:22:14 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

On 05/24/2010 10:19 PM, Anthony Liguori wrote:
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.

If the final destination is a socket, then you end up queuing guest memory as an skbuff. In theory we could do an aio splice to block devices but I don't think that's realistic given our experience with aio changes.

error compiling committee.c: too many arguments to function

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