NFS performance with Citrix Xenserver

Hello!

I am using Citrix Xenserver 5.6FP1 with Celerra NX4. I have created a filesystem on Celerra and exported it via NFS to the Xenserver for use as a VHD repository. Network connection between Celerra and Xenserver is done via two dedicated gigabit switches (Linksys SPS2024). On Celerra, a Fail Safe Network is configured for redundancy, and corresponding bonded link is created on Xenserver. Jumbo frames are enabled on Celerra, the switches and Xenserver.

The filesystem resides on a RAID group of five 450GB 15k SAS disks.

I installed a guest machine with SUSE Linux Enterprise server 11 SP1 and did some simple IO tests with bonnie:

# bonnie -d /home/toomas/bonnie -s 10240

I'm getting block read rates of ca 82 MB/s which seems more or less OK, but block writes are only 25 MB/s, which I feel is somewhat low.

I've tried mounting the filesystem with 'uncached' option and with 'noprefetch' option, which as I read helps performance with VMWare, but with Xenserver it actually makes things slower. Are there any other obvious things I might be missing when it comes to NFS performance?