Gluster users - Jul 2010 - Question About GlusterFS Backed Xen DomU's

Hi Everyone, 

Hope your day has been going well. I have a quick question for everyone.
I am currently using GlusterFS as a distributed SAN backend for the Xen
based cloud platform we are developing. 

We deploy Xen virtuals on pairs of servers using GlusterFs V3 in
replicate mode on Debian Stable (Lenny) with Xen 3.2.1 as the
hypervisor. I am currently experiencing a weird issue where within the
virtual machines (DomU) running on a GlusterFS mount only receive around
10-18MB/s write speeds, but full speed reads. 

Our hardware for each node is Dual Core Xeon Processors, 8GB of RAM and
4 * High Speed SATA drives (RAID 10, around 160MB/s writes and reads). 

If I write a file to the Gluster mount in the Dom0 (host) we receive
around 90-100MB/s writes (maxing out the GigE link). If I run the
virtual machine on the disks without Gluster I get much higher speeds
within the DomU of around 80-90MB/s. 

This slow down only appears to occur on writes. Does anyone with a
better understanding of GlusterFS, Fuse and filesystems have an idea why
this is slowing down. The underlying file system is Ext3. 

Many Thanks
Sheng