Incredibly slow transfer speeds? The NAS may not be at fault
A cautionary tale for all you new users (and existing users alike, I already owned an Ultra 2 which never had this problem with ANY of my home machines!)
Last Friday I began a file copy to a new Duo v2 NAs at work from my workstation (Mid-2011 iMac running a new installation of Windows 7 x64). Gigabit switch, gigabit NIC in the computer, MTU of 9000 set and confirmed operational.
I expected speeds not too far from my home Ultra 2... But the speeds I saw can only be described as crawling pace. 800 - 1600 kB/sec, via FTP! SMB wasn't any faster. After much consternation, gnashing of teeth, rebuilding of arrays, reformatting and resetting to factory defaults, starting over from scratch - none of this worked.
Out of frustration, I began to run a couple of large file downloads (as the office has a 100 Mbps cable connection); after it ramped up to a much faster speed than ever seen with the local copying, I knew something was amiss. I started copying as much as I could to the device at once - and lo and behold, its write speed increased dramatically to what I originally expected it to be.
Bizarrely, copying two large multi-gigabyte test files whilst simultaneously running the FTP transfer, all the transfers zoomed along at almost 50 Mbps - the device's expected maximum write rate for an X-RAID2 array. As soon as the test file transfers were stopped the FTP transfer speed dipped to its low previous rate.
After doing some digging... It's a feature on the NIC in this device, a Broadcom NetXtreme Gigabit Ethernet, causing the problems! Disabling "Large Send Offload v2 (IPv4)" (and its IPv6 equivalent) solved the problem instantly. This problem is apparently well-documented on the web, with many network users suffering from throughput problems and disconnections from network shares until this feature was disabled. So, before you waste a whole weekend, check your NIC settings 😉
Re: Incredibly slow transfer speeds? The NAS may not be at f
The offloading feature on the Broadcom generally works fine with standard ethernet.
Getting jumbo frames to work is often challenging. Generally if you are having poor performance, one of the first things to try is turning them off, and see if it changes things. If it does, then then playing with NIC settings will sometimes fix the performance problem (as you found in this case).
