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Date: Sun, 02 Aug 2009 17:27:14 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] RFC: Add new block driver for the VDI format
References: <4A4E5AFC.4020206@mail.berlios.de> <4A51FE07.2040307@codemonkey.ws>
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Christoph Hellwig <hch@lst.de>

On 07/06/2009 04:37 PM, Anthony Liguori wrote:
>
> I'd really like to get rid of synchronous IO functions in the block 
> layer.  One way to do this is to insist that all new block drivers 
> only implement the AIO functions.
>
> I think we should make this decree but I'd like to know if other 
> people think this is unreasonable first.  One potential model of block 
> drivers would involve synchronous IO and threads.  I'm not a big fan 
> of that model and I don't think it's an easy conversion from today's 
> synchronous IO drivers to that model because the locking and 
> re-entrance needs careful consideration.
>

I agree that sync+threads is not easy, but well performing async is 
much, much harder.  Consider that qcow2 still has synchronous 
operations, and that eliminating the RMW when writing a partial cluster 
concurrently (a very common operation with 64K clusters) is very hard to 
do ayncly and much easier syncly.

Given in addition the large numbers of format drivers, I think we should 
prefer sync+threads over trying to convert all format drivers to full async.

-- 
error compiling committee.c: too many arguments to function