I also get great read speeds from NFS shares (on my nas, check my first post).
Local file copy is fine though.
I setup a CIFS share on the nas and proceeded to copy a 1.2 gig file over CIFS and NFS (to different folders, though they reside in the same ZFS pool).
The NFS transfer completed but would lock up for a bit then burst write.
The CIFS transfer went fine. It did not lock up, the percentage bar steadily moved forward, and for the first time, I feel like I had an accurate write speed displayed through nautilus.
The server showed 30% memory usage and 30% load average (dual core proc) during both transfers.
I've attached another network graph. The CIFS transfer is first and looks much less erratic than the NFS transfer.
I am fairly sure this problem occurred in 9.10 with less frequency. I transferred a terabyte or two to the nas in only a few separate transfers under 9.10. I feel like it would have taken many more transfers under 10.04.
I have tried messing with mtu as well as rsize and wsize. Doesn't help.
The only other export options I can think to change are sync and async, but I have not been able to test this as FreeNAS does not seem to allow this change.
This is all I have for now.