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Date: Tue, 25 May 2010 09:02:53 -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, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, ceph-devel@vger.kernel.org, Christian Brunner <chb@muc.de>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

On 05/25/2010 08:57 AM, Avi Kivity wrote:
> On 05/25/2010 04:54 PM, Anthony Liguori wrote:
>> On 05/25/2010 08:36 AM, Avi Kivity wrote:
>>>
>>> We'd need a kernel-level generic snapshot API for this eventually.
>>>
>>>> or (2) implement BUSE to complement FUSE and CUSE to enable proper 
>>>> userspace block devices.
>>>
>>> Likely slow due do lots of copying.  Also needs a snapshot API.
>>
>> The kernel could use splice.
>
> Still can't make guest memory appear in (A)BUSE process memory without 
> either mmu tricks (vmsplice in reverse) or a copy.  May be workable 
> for an (A)BUSE driver that talks over a network, and thus can splice() 
> its way out.

splice() actually takes offset parameter so it may be possible to treat 
that offset parameter as a file offset.  That would essentially allow 
you to implement a splice() based thread pool where splice() replaces 
preadv/pwritev.

It's not quite linux-aio, but it should take you pretty far.   I think 
the main point is that the problem of allowing block plugins to qemu is 
the same as block plugins for the kernel.  The kernel doesn't provide a 
stable interface (and we probably can't for the same reasons) and it's 
generally discourage from a code quality perspective.

That said, making an external program work well as a block backend is 
identical to making userspace block devices fast.

Regards,

Anthony Liguori