Re: [Xen-users] Xen HVMs run VERY slowly on SAN box

["Fajar A. Nugraha" <list@xxxxxxxxx>]

On Wed, Dec 14, 2011 at 12:07 AM, Benjamin Weaver
<benjamin.weaver@xxxxxxxxxxxxx> wrote:
> We are running a 2-node cluster with Debian Squeeze (2.6.32-5-xen-amd64) and
> Xen 4.0.1 on the dom0 and Ubuntu lucid domUs. We are now using fully
> virtualized (HVM) vms. In our configuration the dom0 administers to domUs
> created and run on a SAN box accessed via private network (192.168...). A
> sample configuration file is pasted below.
>
> We have had little or no IO problem with HVMs created and run locally on
> dom0. However, HVMs run on the SAN suffer from VERY feeble IO. Even a entry
> of a single character or entering return will cause the command line to
> freeze for 30 seconds or more.
>
> We have had no IO problems running 'regular' para-virtualised Xen vms on the
> SAN box. (We are choosing HVMs for separate reasons).

is it the SAME SAN box serving both HVM and PV?

>
> As I understand it, perhaps the problem lies with HVM architecture--IO
> having to travel into dom0 before it is handled by the hardware emulator(?).
> But I am not sure why this would slow IO so much.

It shouldn't.

>
> Are there any parameters we should adjust to enable faster IO for HVMs
> administered on a SAN box?

I'd start by testing if it's REALLY HVM problem. Something like dd/fio
on the dom0, PV domU, and HVM domU to the same path/LUN.
After that, make sure you have PV drivers loaded (exact procedure
depends on what OS you have as domU).

> disk = [ 'file:/ocfs2SAN/lucidxentest3.phon.img,hda,w',

ocfs2 (or any cluster filesystem) will inherently be slower than local
fs (e.g. ext4) or block device. You can try using tap:aio (or
tap:tapdisk:aio, or tap2:tapdisk:aio, whichever works on your version
of Xen) and see if it performs better.

-- 
Fajar

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