[Xen-devel] Re: [PATCH] multi-page blkfront/blkback patch

[Samvel Yuri <samvelox@xxxxxxxxx>]

Thank you very much Geoffrey for sharing blkfront/blkback changes.

Regards,
Sam

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From: Geoffrey Lefebvre <geoffrey@xxxxxxxxx>
To: Samvel Yuri <samvelox@xxxxxxxxx>
Cc: Dutch Meyer <dmeyer@xxxxxxxxx>; Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>; Andrew Warfield <andy@xxxxxxxxx>
Sent: Thursday, November 27, 2008 6:17:58 PM
Subject: [PATCH] multi-page blkfront/blkback patch

Hi,

Here is the refreshed version of the multi-page ring patch for
blkfront and blkback. I currently don't have time to refresh the
blktap part of the patch. The patch should apply cleanly to the
following changeset in the linux 2.6.18 tree:

changeset:  752:0b859c9516ba
tag:        qparent
parent:      751:6591b4869889
parent:      748:5012c470f875
user:        Keir Fraser <keir.fraser@xxxxxxxxxx>
date:        Wed Nov 26 11:13:49 2008 +0000
summary:    Merge with ia64 tree.

The patch adds support for 1,2 and 4 pages ring for block devices. The
number of page is configured when compiling blkfront. The xenstore
setup protocol is backward compatible in the sense that a patched
blkback will support both patched and unpatched blkfront, and a
patched blkfront will use the existing protocol when setting up a 1
page ring so that it works with existing dom0. A guest will fail at
boot time if it tries to setup a multi-page ring with an unpatched
dom0.

The ring setup protocol is a bit crude and it would be much nicer if
the number of ring pages could be configured more dynamically and if
blkfront could adapt to block backend that don't support multi-page
ring (specially if you want to be able to migrate from a patched to an
unpatched dom0). If someone is interested in implementing this
feature, feel free to have a go at it.

The refreshed version of the patch hasn't been tested extensively but
it seems to work well. I don't have access to a setup that allows me
to test if migration works but save/restore seems to work. I also
don't have access to a filer to reproduce the performance number but
the patch did give a significant boost in throughput (from 60 to
100MB/sec) when we last tested it. I am sorry i currently don't have
much time to spend on this but hopefully this will be helpful.

cheers,

geoffrey

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