My drive doesn't run at 40Mb/s on my benchmark program.

The benchmark results you are seeing, around 7-8 megabytes per second, reflects the capability of the drive.

The 40mbs is called the external, or burst, transfer rate. This is the rate at which the drive and controller can pass data between their respective buffers over the cable.

The rate that you are seeing on the benchmark is called a sustained rate. The rate at which the drive can sustain a data transfer over many successive transfers of data, over a period of time.

The rate that will define the sustained data rate is called the internal transfer rate, the rate at which the drive can move data between its platters and its buffer. This is a function of the rotational speed and the recording density.

A normal transfer, a read for instance, involves initially seek time and command overhead, and then the drive will move the data from the correct track into its buffer at the internal transfer rate. Once the buffer is sufficiently full, the data will be transferred over the cable to the controller at the external (burst) transfer rate of 40mbs. The drive must then refill its buffer at the internal transfer rate before it can burst the data to the controller again. A typical large block read will entail a large number of these sequences. The aggregate average of all of the transfers done will give you the figures that the benchmark provides.

An analogy that is often used would be to liken this to a process to a rifle, with the speed of the bullet equating to the burst transfer rate (40mbs), and the internal transfer rate being the speed with which you can reload the rifle prior to firing again. This should give you the understanding that the rate of fire (benchmark results) are more dependant on how fast you can reload, and not how fast the individual bullets travel.

Close this Window