HD Tach will test the sequential read, random access and interface burst speeds of your attached storage device (hard drive, flash drive, removable drive, etc). All drive technologies such as SCSI, IDE/ATA, 1394, USB, SATA and RAID are supported. Test results from HD Tach can be used to confirm manufacturer specs, analyze your system for proper performance, and compare your performance with others. HD Tach is very easy to use, quick, and presents data in easy to read graphs, including the ability to compare two storage devices on screen at the same time for easy analysis.
Bursts are provided only for your review. SSD's don't cache the same way HDD's do (in many cases they don't cache reads at all), so burst testing typically results in figures that are lower than the sequential throughput figures.
HDTach feeds the tested drive a continuous string of small sequential read requests. This is a single threaded operation, which means the SSD doesn't get to see what's coming next. The 840 did great on reads, but the single threaded write pattern (QD=1), combined with the increased write latency of TLC flash, caused this result to be lower than what would happen with a straight file write, which normally takes place with a higher Queue Depth and write IO size.
The Intel 520 beats all on sequential writes, but it uses heavily optimized firmware and is able to compress the pattern written by HDTach, so it sees minimal fragmentation slowdown effects during this test.