Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT

From: Christoph Hellwig <hch@lst.de>
To: qemu-devel@nongnu.org
Cc: Christoph Hellwig <hch@infradead.org>,
	Avi Kivity <avi@redhat.com>,
	kvm@vger.kernel.org
Subject: Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT
Date: Mon, 23 Mar 2009 19:48:27 +0100	[thread overview]
Message-ID: <20090323184827.GA14826@lst.de> (raw)
In-Reply-To: <49C7D096.3000302@codemonkey.ws>

On Mon, Mar 23, 2009 at 01:10:30PM -0500, Anthony Liguori wrote:
> I really dislike having so many APIs.  I'd rather have an aio API that 
> took byte accesses or have pread/pwrite always be emulated with a full 
> sector read/write

I had patches to change the aio API to byte based access, and get rid
of the read/write methods to only have the byte based pread/pwrite
APIs, but thay got obsoleted by Avi's patch to kill the pread/pwrite
ops.  We could put in byte-based AIO without byte-based read/write,
though.  In my patches I put a flag into BlockDriverState whether we
allow byte-based access to this instance or otherwise emulated it in
the block layer.  We still need this as many of the image formats can't
deal with byte-granularity access without read-modify-write cycles,
and I think we're better off having one read-modify-write handler in
the block handler than one per image format that needs it.