Jul 13, 2011

Checking your bandwidth

Imagine this: Company A has a storage server named ginger and it is being NFS-mounted by a client node named beckham. Company A has decided they really want to get more bandwidth out of ginger because they have lots of nodes they want to have NFS mount ginger's shared filesystem.
The most common and cheapest way to do this is to bond two Gigabit ethernet NICs together. This is cheapest because usually you have an extra on-board NIC and an extra port on your switch somewhere.
So they do this. But now the question is: How much bandwidth do they really have?
Gigabit Ethernet has a theoretical limit of 128MBps. Where does that number come from? Well,
1Gb = 1024Mb1024Mb/8 = 128MB; "b" = "bits," "B" = "bytes"
But what is it that we actually see, and what is a good way to measure it? One tool I suggest is iperf. You can grab iperf like this:
# wget http://dast.nlanr.net/Projects/Iperf2.0/iperf-2.0.2.tar.gz
You'll need to install it on a shared filesystem that both ginger and beckham can see. or compile and install on both nodes. I'll compile it in the home directory of the bob user that is viewable on both nodes:
tar zxvf iperf*gz
cd iperf-2.0.2
./configure -prefix=/home/bob/perf
make
make install
On ginger, run:
# /home/bob/perf/bin/iperf -s -f M
This machine will act as the server and print out performance speeds in MBps.
On the beckham node, run:
# /home/bob/perf/bin/iperf -c ginger -P 4 -f M -w 256k -t 60
You'll see output in both screens telling you what the speed is. On a normal server with a Gigabit Ethernet adapter, you will probably see about 112MBps. This is normal as bandwidth is lost in the TCP stack and physical cables. By connecting two servers back-to-back, each with two bonded Ethernet cards, I got about 220MBps.
In reality, what you see with NFS on bonded networks is around 150-160MBps. Still, this gives you a good indication that your bandwidth is going to be about what you'd expect. If you see something much less, then you should check for a problem.
I recently ran into a case in which the bonding driver was used to bond two NICs that used different drivers. The performance was extremely poor, leading to about 20MBps in bandwidth, less than they would have gotten had they not bonded the Ethernet cards together!

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