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

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Message-ID: <20100420200310.GR5660@tracyreed.org>
Date:	Tue, 20 Apr 2010 13:03:10 -0700
From:	Tracy Reed <treed@...raviolet.org>
To:	Pasi Kärkkäinen <pasik@....fi>
Cc:	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 10:54:42PM +0300, Pasi Kärkkäinen spake thusly:
> Please try with "bs=1024k" and maybe with "bs=64k" aswell.
>  
> 4k blocksize transfer will always be slower in domU than in dom0
> since virtual disk abstraction makes some overhead, which is more
> visible with small blocksizes.

But overhead in domU wouldn't be causing all of these reads. I am
doing a test with bs=64k now:

Device:         rrqm/s   wrqm/s   r/s   w/s    rkB/s    wkB/s avgrq-sz
avgqu-sz   await  svctm  %util
sda               0.00  3258.20 964.00 54.00  3903.20 13652.80
34.49     2.75    2.71   0.68  68.88
sdb               0.00  3270.20 974.80 54.00  3940.80 13710.40
34.31     2.42    2.38   0.55  56.22

> So the speed is the same to the partitioned disk than to the raw disk? 
> What disk backend are you using in dom0? phy:? tap:aio: ?

Yes. 

I am using phy: disk backend. Should I be using something else?

-- 
Tracy Reed
http://tracyreed.org

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