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

From: Christoph Hellwig
Subject: Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT
Date: Mon, 23 Mar 2009 13:29:28 -0400
User-agent: Mutt/1.5.18 (2008-05-17)

On Mon, Mar 23, 2009 at 12:14:58PM -0500, Anthony Liguori wrote:
> I'd like to see the O_DIRECT bounce buffering removed in favor of the  
> DMA API bouncing.  Once that happens, raw_read and raw_pread can  
> disappear.  block-raw-posix becomes much simpler.

See my vectored I/O patches for doing the bounce buffering at the
optimal place for the aio path. Note that from my reading of the
qcow/qcow2 code they might send down unaligned requests, which is
something the dma api would not help with.

For the buffered I/O path we will always have to do some sort of buffering
due to all the partition header reading / etc.  And given how that part
isn't performance critical my preference would be to keep doing it in
bdrv_pread/write and guarantee the lowlevel drivers proper alignment.

> We would drop the signaling stuff and have the thread pool use an fd to  
> signal.  The big problem with that right now is that it'll cause a  
> performance regression for certain platforms until we have the IO thread  
> in place.

Talking about signaling, does anyone remember why the Linux signalfd/
eventfd support is only in kvm but not in upstream qemu?

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