RE: [Xen-users] XEN 3.1 on 2.6.18-164.el5xen kernel - Slow IO & Pauses

["Rob Taylor" <rob@xxxxxxxx>]

Well, for the most part I'm using dd if=/dev/zero of=test bs=4k
count=1000000

This writes a 4gig file to the disk.   I've tried with 100M, 200M, 400M,
800M, 1G, 2G, 4G file sizes.
100M-800M the throughput is quite impressive.
1G the throughput is still 100+M/sec.
2G+ it goes down to 1-2M/sec on DomU.

----

I've done the iostat, and in most all cases the most of the usage is in the
"await" state on the drive I'm testing.
I'll keep digging to see if I can find out more information.  Thanks for the
reply.
 

-----Original Message-----
From: Fajar A. Nugraha [mailto:fajar@xxxxxxxxx] 
Sent: Sunday, November 08, 2009 9:01 PM
To: Robert Taylor
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] XEN 3.1 on 2.6.18-164.el5xen kernel - Slow IO &
Pauses

On Sat, Nov 7, 2009 at 2:55 AM, Robert Taylor <rob@xxxxxxxx> wrote:
> I've seen tons of posts about slow IO under Xen and others without 
> slow IO under xen.   Here's what I'm experiencing.   I've got a XEN 
> system on an 2 dual core AMD Opteron 275 processors.
>
> I'm running DRBD for disk, but have also tested without DRBD.
> Throughput in Dom-0 seems to be 50-80MB/sec for a file of any size.
> Throughput in DomU is 20-30MB/sec for a file under 2GB, and 1-2MB/sec 
> for a larger file.

How do you measure the throughput?

> Any help would be appreciated.  This is under CentOS 5.4, 8GB memory, 
> 1TB SATA nv-sata controller.   Tyan K8SRE S2891 motherboard.   NVidia 
> nForce 4Port SATAlink controller.

I'm using a similar software setup (RHEL5), different hardware though.
I'd suggest you start with doing "iostat -mx 3" on both dom0 and domU to see
where the bottleneck is. My guess is that some process is using lots of I/O
(possibly in small sizes) so that you're practically IOPS bound, and the
problem is not directly related to Xen.

--
Fajar


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