PHD Zero Filler will benchmark the write speed of your disks either on a physical host or in a VM as it Zero's out all your empty space. Simply extract the zip file and run PHDZeroFiller.exe. The application writes a file full of nulls as fast as it can in the block size you specify (testing has shown 16384 to be an optimum block size). It will display the current and average write speeds as well as how much it has written. Hit Ctrl-C to stop the benchmark and it will automatically delete the test file. DO NOT RUN THIS ON A GSX VM WITH A GROWABLE FILESYSTEM OR YOU WILL END UP WITH A HUGE VMDK. Running this tool inside of an ESX VM and then running esxtop on the host is a great way to test the disk IO of your ESX server.