I have recently spent a considerable amount of time trying to work out why the read rate of my Netgear ReadyNAS Duo (using CIFS/SMB) was only giving me around 10MB/s. As this (well 12.5 MB/s to be precise) roughly works out at 100Mbps I figured it was clearly a networking issue. I tried all manner of things including playing with cables, network switches, driver versions, Flow Control, Jumbo Frames, TCP Offloading.. all of these had some effect on the transfer but not significantly enough to make the network card work harder than 10% of its full capacity. I have no other gigabit devices on my network so testing was rather tricky but I eventually managed to eliminate everything but the network card and the ReadyNAS.
Others who also had this device were reporting perfect speeds with it, so I became a bit attached to a forum post blaming Windows 7 for giving slow network performance (
Why is Windows 7 so slow in copying network files?) particularly given a large percentage of the users listing their network hardware mentioned "realtek". As a result of one of the posts I even ended up buying a new gigabit network card:
There was a windows update that changed the driver a few months ago and I had problems after that (Realtek driver update for Realtek PCIe GBE Family Controller 10/25/2010).
Then I found a chart that seemed to reproduce my issue quite nicely:

Source:
SmallNetBuilder.com
Others I knew to have a ReadyNAS, so I then asked them to specifically test large files over SMB. They reported that though AFP transfers were fine, they too had an issue with SMB.
With a bit of searching, in the end it turned out to be the ReadyNAS' most recent firmware (V4.1.8) that introduced this problem.
Hidden away in the "Performance" section of the ReadyNAS forums is
Performance halved at a stroke (ReadyNAS Duo, 4.1.8) ... and
Performance drop after update to 4.1.8 Duo apparently the issue started somewhere between the last beta of 4.1.8 (T9) and the final release of 4.1.8 (
Lion, AFP, 4.1.8-T9 vs 4.1.8).
The current public beta (test software version) fixes for not only me but also others according to
4.1.9 T2 Performance as for a test version 3 (T3) : "No T3 right now." (
Source). You can download the test version here :
*NEW BETA* RAIDiator 4.1.9 and I believe you should be able to downgrade. The current change log for it is the following:
Bug Fixes since RAIDiator 4.1.8
1. [T2] Fixed performance problem reported by forum members, specifically folks using the ReadyNAS over AFP.
I do not even use AFP, but I certainly found my SMB/CIFS read performance to be improved to a more healthy 29MB/s after trying the second test version of 4.1.9.