The standard program used to run a simple disk I/O benchmark on a UNIX-like system is bonnie++. In its default configuration, it will create a file (or set of files) twice as large as the physical RAM in the server, to prevent the entire test from being cached in RAM, then read that whole file back in again. The main virtue of the program is that you can just run it from a directory on the volume you want to test and it usually does the right thing without further input.
Here's a full example of downloading bonnie++, building it, running it, and getting an HTML report from it:
$ wget http://www.coker.com.au/bonnie++/bonnie++-1.03e.tgz $ tar xvfz bonnie++-1.03e.tgz $ cd bonnie++-1.03e/ $ ./configure $ make $ ./bonnie++ -f -n 0 | tee ...
