linux-kernel - Re: [Xen-devel] domU is causing misaligned disk writes

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Date:	Tue, 20 Apr 2010 14:58:59 -0700
From:	Tracy Reed <treed@...raviolet.org>
To:	pasik@....fi, xen-devel@...ts.xensource.com,
	Aoetools-discuss@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] domU is causing misaligned disk writes

On Tue, Apr 20, 2010 at 01:41:51PM -0700, Brendan Cully spake thusly:
> You could also be limited by the size of the block request ring (I
> believe the ring is normally only one page) -- the ring needs to be
> large enough to handle the bandwidth delay product, and AoE means the
> delay is probably higher than normal.

Interesting. Any easy way to increase this as a test?

> Do you get better performance against a local partition?

You mean a local partition on local disk in the dom0 given to a domU
as xvd? Let's see...

I just created a 20G logical volume on the dom0:

# /usr/sbin/lvcreate -n test -L20G sysvg

Added it to the domain config file to be /dev/xvdi and rebooted.

         "phy:/dev/sysvg/test,xvdi,w"

I know you can attach block devices on the fly but this has not been
entirely reliable for me in the past so I reboot now.

In domU against xvdi which is /dev/sysvg/test in dom0:

# dd if=/dev/zero of=/dev/xvdi bs=4096 count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 99.3749 seconds, 41.2 MB/s

And iostat on dom0 shows:
Device:         rrqm/s   wrqm/s   r/s   w/s    rkB/s    wkB/s avgrq-sz
avgqu-sz   await  svctm  %util
sda               0.00   824.00  4.00 101.00    16.00 39936.00
760.99     3.19   31.31   9.52 100.00

In dom0 against the local disk to demonstrate native performance:

# dd if=/dev/zero of=/dev/sysvg/test bs=4096 count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 84.9047 seconds, 48.2 MB/s

Device:         rrqm/s   wrqm/s   r/s   w/s    rkB/s    wkB/s avgrq-sz
avgqu-sz   await  svctm  %util
sda               7.00 11104.00  5.00 96.00    48.00 48144.00   954.30
133.92 1172.40   9.94 100.40

Virtually no reads happening. This disk seems a bit slow (older 80G
sata disk) but otherwise normal. I don't see anything indicating
alignment issues.

-- 
Tracy Reed
http://tracyreed.org

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