I recently built a fairly nice NAS system for a customer:
0.7 Khasadar (revision 4919)
2.5GHz Dual Core Pentium
4GB DDR2 PC1066
5x hot swap 2TB SATA
10/100/1000 Intel Pro NIC
10/100/1000 Full duplex switch connection

After formatting the drives in SoftRAID 5 (1.8TB x 4 = 7.2TB – 5% for defragmentation = 6.84TB available), I started to backup many of their ISOs and daily computer backups and other logs. Everything was going smoothly using Windows File Sharing (Samba) until I got to the larger files.

Initially the files would copy at 80+MB/sec, but then they’d slow down to 50… 30… 20… even down to 8 or so MB/sec. Ouchies when you’re talking about a 40+GB file to transfer. FTP yielded the same results. I noticed that sometimes the drives on the NAS wouldn’t even be blinking, so it had to be another issue.

If you login on the web panel of the FreeNAS, under Services you will find CIFS/SMB.
Check under Advanced Settings
You’ll see Send and Receive buffer sizes – by default they are 16MB (16*1024 = 16384)
I changed the buffer size to 128MB (128*1024 = 131072) and checked the results

120MB/sec, slow down to 80… 70… 60… So 60MB/sec. That’s over 6 times faster at a cost of 8 times the RAM. I can live with that.

So just for jollies I increased it to 512MB and then to 1024MB. Both stopped around 70MB/sec. Then, as a final test, I dropped it down to 256MB (256*1024 = 262144), which seemed to be OK.