CIFS/iSCSI OK, NFS writes really slow! [Fixed - Use ASYNC]

Hi all, newbie with a ReadyNAS 102 having a really bad time with NFS write performance.

Just got a 102 model for christmas and I will admit I have been giving it a damn good testing. Day one, firmware update went on (6.1.4) and I had a play about generally to get an understanding. Making and breaking raid volumes etc, managed to setup CIFS iSCSI and NFS. I got what I expected performance wise for iSCSI and CIFS (30/40MB/sec write), though i did break iscsi. But NFS writes where just well unbelievable using a simple dd if=/dev/zero of=/mnt/"share"/output bs=4k count=100k performance test I seemed to get less than 10MB/sec! It really didn't like Compression being turned on.

Today, I have gone round the houses to try and find out what I am doing wrong. First step was firmware update to 6.1.5, and then factory reset to get all the update features (and remove any marks from past testing). Waited patiently for the Volume to rebuild before performance testing.

Background and other kit..
ReadyNAS102 has 2 500GB Hitachi SATA disks 7200RPM (pulled from a working system) - No Apps installed only services on SMB NFS HTTP HTTPS SSH (yes i am a bad person) - one data volume factory default X-Raid mirror.
Procurve J4850A with flow control on, no jumbo frames (Xen won't let you use jumbos on management adapter)
3 x 6.2 Xen Servers 2.6Ghz G620 intels with 8GB ram and a local 500GB SATA.
Debian 7.0 Virtual machine one on Xen and one on windows8 in oracle Virtualbox.
Stonking big Windows 8 machine

First tests, SSH on to the readynas run a DD test.. DAMN fast as expected! So disks seem ok.

root@ReadyNAS01:/data# dd if=/dev/zero of=/data/output bs=4k count=100k; rm -f /data/output
102400+0 records in
102400+0 records out
419430400 bytes (419 MB) copied, 3.21765 s, 130 MB/s

So CIFS from Windows8 to a share with CIFS and NFS on. Using NAS performance tester 1.5, found a post here referring to it. And yes nice results procurve indicate 30% to 40% bandwidth usage.

Running warmup...
Running a 400MB file write on \\172.16.1.61\XEN-NFS-01 5 times...
Iteration 1:     44.36 MB/sec
Iteration 2:     47.72 MB/sec
Iteration 3:     43.43 MB/sec
Iteration 4:     42.81 MB/sec
Iteration 5:     44.54 MB/sec
------------------------------
Average (W):     44.57 MB/sec
------------------------------
Running a 400MB file read on \\172.16.1.61\XEN-NFS-01 5 times...
Iteration 1:     91.87 MB/sec
Iteration 2:     85.33 MB/sec
Iteration 3:     86.24 MB/sec
Iteration 4:     77.04 MB/sec
Iteration 5:     88.23 MB/sec
------------------------------
Average (R):     85.74 MB/sec
------------------------------

Almost text book, read rate double the write (raid1). With flow control and no jumbo packets I didn't expect to break more than 60MB/s read. Conclude all is well a windows file copy bobs round 40MB/s.

Same share mounted using NFS from either a Debian 7 or Xenserver as an SR with a VM running on it writes less than 10MB/s. (fresh installs no tweaks all default)

root@debsvr1://# mount 172.16.1.61:/data/XEN-NFS-01 /mnt
root@debsvr1://# dd if=/dev/zero of=/mnt/output bs=4k count=100k; rm -f /mnt/output
102400+0 records in
102400+0 records out
419430400 bytes (419 MB) copied, 208.548 s, 2.0 MB/s

BUT... If you do a short burst write I get 34.8MB/s (more what I would expect)

root@debsvr1://# dd if=/dev/zero of=/mnt/output bs=4k count=10k; rm -f /mnt/output
10240+0 records in
10240+0 records out
41943040 bytes (42 MB) copied, 1.20669 s, 34.8 MB/s

So what about Debian 7 from Xen to Debian 7 on Virtualbox?

Export /media/DATA1 172.16.1.217(rw,sync)

root@debsvr1://# mount 172.16.1.75:/media/DATA1 /mnt
root@debsvr1://# dd if=/dev/zero of=/mnt/output bs=4k count=100k; rm -f /mnt/output
102400+0 records in
102400+0 records out
419430400 bytes (419 MB) copied, 13.6585 s, 30.7 MB/s

I have tried increasing the NFS Threads to 16 and 32 with no noticeable change.. The NFS share on the ReadyNAS has compression off, AV off, Continuous Protection off, Any host READ/WRITE, Async off (no UPS must play safe). Security defaults, no users added and the folder is set as factory default.

So I am a bit lost! I have had a hit miss relationship with NFS over the years. When it works, it works maybe not as fast as some people claim but good enough (like 30.7MB/s write). So any ideas on how to get the writes out of the gutter of sub 10MB/s?