Re: [Xen-users] NFS DomU Client Performance

Martin Dziobek
Thu, 24 Aug 2006 16:55:27 -0000

Hi Dan,

I suspect thats the old MTU/fragmenting problem.
Try to mount with option tcp (-o tcp).

As far as I recall, udp packets larger than (1500-something)
are transmitted in fragments and those are not reassembled
correctly by xen.
You can track this down by setting rsize=,wsize= to lower
values,too. There is a limit somewhere at 14xx.

Surprisingly, nobody at xen''s seems to care about this...

Hope it helps,

Martin

On Wed, 23 Aug 2006 13:28:08 +0100
"HAWKER, Dan" <dan.hawker@astrium.eads.net> wrote:

>
>
> Hi All,
>
> Quick question regarding NFS performance and Xen.
>
> Have a series of Xen boxes running Xen 3.0.2 on FC5 with a stock FC5 Xen0/U
> 2.6.16 kernel. All things generally are great.
>
> Was planning to deploy a couple of VMs for a couple of our software
> developers so that they can test new software in relative safety, however I
> am experiencing some poor performance issues with NFS.
>
> NFSServer1 - Standard FC5 box (ie no Xen)
> NFSServer2 - Xen based DomU FC5 box (exactly the same as above but runs as
a
> DomU)
>
> DevClient1 - Standard FC5 box
> DevClient2 - Xen DomU FC5 box
>
> Basically, if I mount a NFS share from either NFSServers on DevClient2 it
> takes a while to mount, but will eventually mount fine. If however I try to
> mount a share on DevClient1 it mounts instantaneously. I have tested this
on
> a number of DomU boxes I have around for other services (LDAP/DNS/etc) with
> similar results. Equally if I mount shares from a number of non-Xen boxes
it
> is again instantaneous.
>
> Now I was expecting some overhead with Xen networking (as you''d
expect) but
> this is a bit more that I was expecting. From my initial testing
(admittedly
> probably a little rough), a simple *mount nfsserver1:/mountpoint
> /mnt/mountpoint* sits there for a good 90 seconds (although the end result
> is it does mount), whereas a non Xen client mounts in under a second.
>
> It doesn''t seem to make any difference whether the NFSServer is a
Xen or
> non-Xen environment, the limiting factor always seems to be the Xen Client.
>
> Initially I thought this maybe DNS issues, but this doesn''t seem
to be the
> case. I have digged around and all is normal with correct results from any
> machine.
>
> Any suggestions, is this a known problem, etc, etc??
>
> TIA
>
> Dan
> --
>
> Dan Hawker
> Linux System Administrator
> EADS Astrium
>
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