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Re: [Qemu-devel] buffer alignment for block backends

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] buffer alignment for block backends
Date: Thu, 09 Apr 2009 17:50:19 +0200
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On 04/09/09 17:21, Samuel Thibault wrote:
Gerd Hoffmann, le Thu 09 Apr 2009 15:54:34 +0200, a écrit :
On 04/09/09 15:30, Stefano Stabellini wrote:
The userspace\kernelspace distinction does not make much sense in
minios, but yes it provides an interface to communicate with blkback in
dom0. This interface is used by block-vbd in qemu instead of
block-raw-posix when run in minios.
Sit back, look again.  Do you really need it being page-aligned?

Page aligned is an optimization which comes for free when you can
already choose the alignment.

Ok, so you don't have to.

Does it have an effect positive actually? i.e. do you have requests which would fit into one page but actually span two due to the misalignment?

At least when watching the linux kernel access pattern I see larger requests being page aligned anyway. Probably page cache reads and writes.

The xen block protocol can handle unaligned requests just fine.

Not completely arbitrarily aligned requests. They need to be

Agreed. Sector-alignment you have anyway, thats why I didn't wrote that explicitly.


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