From the Xdd User's Guide:

About xdd

Xdd is a tool for measuring and characterizing disk subsystem I/O
on single systems and clusters of systems. It is a command-line
based tool that grew out of the UNIX world and has been ported to
run in Windows environments as well. It is designed to provide
consistent and reproducible performance measurements of disk I/O
traffic. There are three basic components to xdd that include the
xdd program itself, a timeserver program, and a gettime program.
The timeserver and gettime programs are used to synchronize the
clocks of xdd programs simultaneously running across multiple
computer systems.

WWW: http://www.ioperformance.com/