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

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Andrej Radonic
> Sent: 22 June 2007 10:41
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] Poor disk io performance in domUs
> 
> 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
I take it you mean "two parallel 'dd' commands at the same time"? That
would still write to the same portion of disk (unless you specifically
choose different partitions?)

If it's the same partition, then, althouth there is some head movement
involved, there will be less head movement than two domains that start
10GB apart on a disk. Also, the filesystem driver in the Dom0 can
re-arrange the disk accesses to make fewer movements. 

> dd simultaneously in two domU = 34 MB/s
I take it this means two different DomU doing "dd"? 
Is that 34 MB/s "total" (i.e. 17MB/s per domain) or per domain (68 MB/s
total)? 

> dd in a single dom0 = 120 MB/s
So this is "better" than half of 170 MB/s, agreed? So even in a single
domain, running parallel sessions reduce the performance. 

Note also that there is overhead in transferring from Dom0 to DomU and
the other way around. Even if this is fairly small, it's not possible to
ignore this. 

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

It's difficult to say - I'm just trying to give you some explanation to
what you're seeing. 

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