Generating Constant Bandwidth on Linux using fio

It took a lot of searching for me to find a way to generate network traffic at a specific rate between two hosts, so I thought I would share the answer. It's pretty easy to test the available bandwidth between two hosts using netcat to transfer a bunch of random data as fast as the network allows. However I wanted to test resource monitoring and graphing system, which means I needed to generate network traffic at a known rate so that I could judge the resulting graphs against my expectations.

I found you can use fio, which is a generic I/O testing tool, to achieve this. fio allows specifying the transfer rate and also has a network engine. So using fio I can configure one host as the receiver and one as the sender and transfer data at a known rate. Here's what the config files look like:

Sender jobfile.ini:

[test1]
filename=receiver.dns.name/7777
rw=write
rate=750k
size=100M
ioengine=net

Receiver jobfile.ini:

[test1]
filename=localhost/7777
rw=read
size=100M
ioengine=net

Obviously you would replace "receiver.dns.name" with the DNS name of the receiving host, and adjust the size and rate parameters as you like. (It's worth noting that the fio documentation is either wrong or misleading on what the filename should be for the receiver. It claims the receiver should only specify the port, but when I tried that it failed to run. Setting the host to localhost seemed to work and the receiver started listening.) To run the test, simply run:

fio jobfile.ini

first on the receiving host, then on the sending host. fio will then transfer 100 Megabytes of data at a rate of 750KB/sec between the two hosts. And we can see from the chart that indeed a constant rate was generated:

The observed rate is a bit above the 750KB/sec specified, but what's being measured is the number of bytes being transferred through the eth0 interface. Since the data is transferred over TCP there is some overhead to the packet structure, which I believe accounts for the extra few KB/sec observed.