> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Andrej Radonic
> Sent: 22 June 2007 08:57
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Poor disk io performance in domUs
>
> Tom,
>
> > Are the disk I/O numbers quoted below aggregate for all
> machines, or
> > just for the mailserver.
>
> 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].
--
Mats
>
> Andrej
>
> > -- -- Tom Mornini, CTO -- Engine Yard, Ruby on Rails
> Hosting -- Support, Scalability, Reliability -- (866)
> 518-YARD (9273) On Jun 20, 2007, at 10:49 PM, Andrej Radonic wrote:
> >> > Hi,
> >> >
> >> > in a previously well behaving xen setup we are suddenly
> >> > experiencing massive loss of disk performance.
> >> >
> >> > Setup: two identical machines (dell, 2 quadcore cpus
> each, 8 GB RAM
> >> > each), attached to *one* SAS storagebox (Raid5 SAS
> disks), CentOS
> >> > 5, Xen 3.04 running three domUs each.
> >> > Storage partitions are exposed as block-devices to the guests.
> >> > Filesystem is ext3.
> >> >
> >> > Since a domU running a rather heavily loaded mailserver
> (many small
> >> > files, many reads/writes) has been added, io performance has
> >> > deteriorated significantly.
> >> >
> >> > In dom0 about 100 MB/s can be achieved, in domU only
> about 50 MB/s.
> >> > This looks even worse when there are massive concurrent
> / parallel
> >> > io jobs, then throughput goes down to 20 MB/s.
> >> >
> >> > During all this the cpus are mostly idle...
> >> >
> >> > Any ideas? I know some performance degradation is to be
> expected
> >> > but shurely not on this scale?
> >> > Thanks,
> >> > Andrej
> >> >
> >> >
> >> > _______________________________________________
> >> > Xen-users mailing list
> >> > Xen-users@xxxxxxxxxxxxxxxxxxx
> >> > http://lists.xensource.com/xen-users
>
>
>
> interSales AG Internet Commerce
> Andrej Radonic
> Vorstand
>
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>
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