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Date: Mon, 26 Oct 2015 16:37:29 +0100
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Subject: Re: [Qemu-devel] 4k seq read splitting for virtio-blk - possible
	workarounds?
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To: Andrey Korolyov <andrey@xdel.ru>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: Peter Lieven <pl@kamp.de>



On 26/10/2015 12:50, Andrey Korolyov wrote:
> Hi,
> 
> during the test against generic storage backend with NBD frontend we
> found that the virtio block device is always splitting a single read
> range request to 4k ones, bringing the overall performance of the
> sequential reads far below virtio-scsi. Random reads are going
> relatively well on small blocks due to small overhead comparing to
> sequential ones and writes are ok in all cases. Multiread slightly
> improves the situation, but it would be nice to see complete
> pass-through of range read requests down to backend without an
> intermediate splitting.
> 
> Samples measured on an NBD backend during 128k sequential reads for
> both virtio-blk and virtio-scsi are attached. Please let me know if it
> looks like that I missed something or this behavior is plainly wrong.

How does the blktrace look like in the guest?

Paolo