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 
> Peter Braun
> Sent: 23 June 2007 11:52
> To: Xen users mailing list
> Subject: Re: [Xen-users] Poor disk io performance in domUs
> 
> Lets say we have 4 domU heavy utilized.
> 
> What about providing physical disk to every domU to avoid sharing
> single disk among all guests?
> 
> Will it help to have didecated disk per domU?

Yes, that will help to some extent. You still have to consider that Dom0
is handling all the disk IO. 

--
Mats
> 
> 
> Peter
> 
> 
> > 2007/6/22, 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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> > > > Xen-users@xxxxxxxxxxxxxxxxxxx
> > > > http://lists.xensource.com/xen-users
> > > >
> > > >
> > > >
> > >
> > >
> > >
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> > >
> >
> 
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