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

[Andrej Radonic <rado@xxxxxxxxxxxxx>]

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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