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Re: [Xen-devel] Strange thing:DomU's I/O performance in harddisk is bett

To: ???? <zhouyejoe@xxxxxxxxxxx>
Subject: Re: [Xen-devel] Strange thing:DomU's I/O performance in harddisk is better than Dom0
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Tue, 22 Dec 2009 10:47:48 +0200
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On Tue, Dec 22, 2009 at 07:09:23AM +0000, ???? wrote:
>    Hi, all:
>         I have done some experiments to test the write/read harddisk speed in
>    Dom0 and DomU in order to find out that DomU's filesystem is ext2 or ext3,
>    and I got  strange data:DomU's speed is faster than Dom0.
>         I use IOZONE to do my test, and I think the data showed can be exact.
>    Dom0's kernel is 2.6.18 and xen 3.2.0 and  DomU's OS is on a separate
>    harddisk partition. In Dom0 with ext3 file system type the write speed is
>    assumbly 80M/s, with ext2 file is 90M/s. But in DomU it showed 105M/s.And
>    I still don't know DomU's file system type, ext2 or ext3?
>       Thanks,all!

Are you using file: disk backend in /etc/xen/<guest> cfgfile? 

file: backend uses dom0 kernel memory caching, so there's more caching for the 
domU than for dom0, 
so that usually explains why domU seems faster than dom0. Also, you
should always benchmark with dataset size of 2-3x of your RAM to mask
out the caching effects.

Try using tap:aio: backend, or change to LVM volumes and phy: backend.

-- Pasi

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