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RE: [Xen-users] Poor disk io performance in domUs

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Poor disk io performance in domUs
From: Andrej Radonic <rado@xxxxxxxxxxxxx>
Date: Fri, 22 Jun 2007 11:40:31 +0200
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Hi Mats,

so far we measured separately in dom0 and in the mail-domU.
> The 20 MB/s happen as soon as there is concurrent io from two > or more domUs.

That would be a consequence of the two domains causing more
head-movement on the drive(s) than a single domain, so you get more
overhead. So if you get 50MB/s in a single domain, you don't get 25MB/s
in two parallel domains - you get a bit less. That's just what I'd
expect in this situation. [This is because the "disk" for each virtual
machine is in a different part of the disk, so each time the first
domain acesses the disk, it needs a (big) move of the read/write head,
and then another big move when the second domain accesses it's part of
the disk].

OK, I definitely agree, but I am still not sure about the extent of throughput degradation.

I remeasured the setup. This is what I get using dd to write a large file to the disk:

dd simultaneously in both dom0 = 170 MB/s
dd simultaneously in two domU = 34 MB/s
dd in a single dom0 = 120 MB/s

Would you really say that one-third of io performance is what is to be expected?

Thanks for your cooperation.
Andrej
interSales AG, Cologne/Germany

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