[Xen-devel] Re: poor domU VBD performance.

[peter bier <peter_bier@xxxxxx>]

Ian Pratt <m+Ian.Pratt <at> cl.cam.ac.uk> writes:

> 
> > I found out that dom0 does file-system IO and raw IO ( using 
> > dd as a tool to test
> > throughput from the disk ) is about exactly the same as when 
> > using a standard 
> > linux kernel without XEN. But the raw IO from DomU to an 
> > unused disk ( a second
> > disk in the system ) is limited to fourty percent of the 
> > speed I get within Dom0.
> 
> Just to be clear: you're doing a dd performance test within dom0 to the
> exact same partition on the 2nd disk that you're using when you start
> the domU and finding that the domU 'dd' performance is 40% of the dom0
> performance?
> 
> I've not heard of anyone else having problems like this. What happens if
> you use a partition on the 1st disk?
> 
> What chipset is the IDE controller? What device (e.g. sda1) are you
> exporting the disk partition into the domU as?
> 
> Are you sure dom 0 is idle when doing the dd test in the domU?
> 
> Ian
> 
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> 

Yes, I have tried various partitions both from Dom0 and DomU on both disks and
the result has always been a performance ratio of 2.5 between Dom0 and DomU. 
Yes I used dd for the test. But I came accross this problem doing IO into 
the filesystem. I was surprised that I did not only get no improvement when
switching for a loopbacked file as "device" for DomU to a real device but that
I got a performance degradation. 
With that effect I started to test raw io performance using dd.

I am sure that the device was not busy and dom0 was idle when I did the test. 
There where no busy jobs in dom0 neither CPU- nor IO-bound. I don't know
which chipset the ide-controller is. My mainbord is a MSI KT7 board. I am
currently not at home, must lookup what the ide-controller is. 

The devices I exported to have been hda1 and hdb6 on my computer at home and
hdg5 in the office. In the latter case the disk is attached to a Promise202
raid controller. 

Is there any description what I have to do to configure my system adequately 
to run efficiently using Xen ? If such where available I might be able to 
locate the problem myself.

I have not yet done a "dd performance" test using loopbacked files as devices
yet. I only used them as filesystems. 

Thanks in advance 
          Peter Bier 





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