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

["Peter Braun" <xenware@xxxxxxxxx>]

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?


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
> > >
> > > _______________________________________________
> > > Xen-users mailing list
> > > Xen-users@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-users
> > >
> > >
> > >
> >
> >
> >
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> >
_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users