Xen devel - Dec 2009 - Strange thing:DomU''s I/O performance in harddisk is better than Dom0

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! 

     
 		 	   		  
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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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