HD Tach is a physical performance hard drive test that uses a special kernel mode VXD to get maximum accuracy by bypassing the file system.

The HD Tach sequential read test is a little bit different from other benchmarks. Most benchmarks create a file on the hard drive and test within that file. The problem is that modern hard drives use a zone bit recording technique that allows different read speeds depending on where the data is located. Data on the outside of the drive is much faster than data recorded on the inside.

Obviously for SSDs this is not an issue. We reach the advertised performance quite well at roughly 210MB/sec measured.

In addition to sequential read performance, HD Tach tests the drive's random access time. Random access is the true measure of seek speed. Many drives advertise sub 10 millisecond seek speeds, but seek speeds are misleading. Also again, without a physical head that needs moving around, the SSDs are just soooo fast as they have no mechanical moving parts. Here obviously .. the smaller number is better, 0.1 ms (!).

Here's a little test report of HDTach in both a long READ and WRITE sequence as well. Really fast results, slightly slower than advertised by OCZ. That write performance is sustained though, making it really really fast performance.