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[Xen-devel] [PATCH] Blktap update (RFC)

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] [PATCH] Blktap update (RFC)
From: Julian Chesterfield <jac90@xxxxxxxxx>
Date: Sat, 8 Jul 2006 02:48:57 +0100
Cc: andrew.warfield@xxxxxxxxxxxx, Julian Chesterfield <jac90@xxxxxxxxxxxx>
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Attached is an updated patch to the blktap kernel driver and userspace toolset. The patch incorporates comments submitted to the list regarding stylistic changes such as the use of C99 initialisation. The /proc stats have been removed and will be replaced soon with sysfs variables along the same lines as the recent blkback patch.

This patch incorporates 3 new features:

- It now allows local mounts of tap-based images within dom0 (equivalent to the loopback driver) using the xm block-attach feature. Tap disks can be attached to a virtual block device in dom0 which is then mountable. This feature is particularly useful for modifying VM images before booting them, for instance to update the in-image kernel for an HVM guest.

- Faster Qcow performance through metadata allocation optimizations. Qcow files remain backwards compatible, but headers contain additional information to indicate that allocation has been optimized for access from the xen tools.

- the blktapctrl daemon is launched automatically along with xenstored and xenconsoled through xend

Performance results indicate that blktap performs very favourably in comparison to blkback (raw partition AIO) and that with the qcow optimisation feature enabled, the CoW disk performs almost as fast as the raw loopback driver:

Bonnie results (block IO summarised):
Tap AIO partition 52781 45072
Blkback partition 52596 45429
Loop 52286 37672
Qcow Normal 43058 29386
Qcow Optimised 51834 37083

Full Bonnie results:
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
Tap AIO dev 2G 43805 93 45072 8 21896 0 38751 52 52781 0 121.1 0
Blkback 2G 43400 86 45429 8 22217 0 39075 53 52596 0 119.0 0
Loop 2G 31459 63 37672 9 16442 0 44122 78 52286 0 119.7 0

Qcow norm 2G 28341 56 29386 5 15908 0 36006 49 43058 0 66.9 0
Qcow Opt 2G 33568 67 37083 6 19589 0 39180 58 51834 0 123.1 0


Attachment: blktap.patch
Description: Binary data

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