[CentOS-virt] io writes very slow when using vmware server

Tue Oct 7 17:42:54 UTC 2008
Pasi Kärkkäinen <pasik at iki.fi>

On Tue, Oct 07, 2008 at 10:26:47AM +0200, Paul Bijnens wrote:
> On 2008-10-06 10:39, Pasi Kärkkäinen wrote:
> >On Sun, Oct 05, 2008 at 11:28:45AM +0200, Paul Bijnens wrote:
> >>We are struggling with a strange problem.
> >>When we have some VMWare clients running (mostly MS windows clients),
> >>than the IO-write performance on the host becomes very bad.
> >>The guest os's do not do anything, just having them started,
> >>sitting at the login prompt, is enough to trigger the problem.
> >>
> >>The host has plenty of 4G of RAM, and all clients fit easily into
> >>the space.
> >>The disksystem is a RAID5 (ok, not the fastest choice).
> >>
> >>Both hosts are running the latest CentOS 5.2 (but the problem was
> >>already present in 5.0 as well), and running the latest production
> >>version of VMWare server 1.0.7 (but when running 1.0.5 the problem
> >>was there too).
> >>
> >
> >Hello.
> >
> >I assume you know VMware server v1.x is not supported on RHEL5/CentOS5.. 
> >
> >Have you tried VMware server v2.0?
> You wouldn't believe now many times i've checked to see if 2.0 was
> in production.  I did not verify before sending this mail.
> Also, because it was rather difficult to test beta software on these
> servers (only limited time in the weekends were free to test variations
> in the setup), I did not try to run 2.0beta.
> To my surprise indeed 2.0 is now out of beta indeed. I installed it,
> and indeed, it works!  No performance loss, even with 4 VM guests
> running.
> Case closed.

> >20 MB/sec sounds really bad. There has to be something wrong with your
> >raid/disk setup.
> I don't believe 20MB/s sustained write performance is too bad for RAID5
> with 4 sata disks, in a inexpensive machine.  Do you do much better?

Not me, but:

4 disk raid5:
sequential writes: 297 MB/sec
sequential reads: 407 MB/sec

I remember seeing >100 MB/sec sequential reads and writes with at least 4 years old
hardware and 4x sata-disk (7200 rpm) software md-raid5.

-- Pasi