Physical Drive Performance
Here I will discuss drive performance measurements and physical/mechanical factors that influence them.
Probably the two most important measurements when evaluating hard disk performance are seek time and transfer rate. You may also see access time quoted. These terms will be discussed here.
Seek time
The seek time (usually quoted as the average seek time) refers to the average amount of time it takes to move the drive head from one random track on the platter to another. Sometimes 'track-to-track' seek times are quoted, which refer to the time taken to move from one track to the next adjacent track. The lower the seek time, the better. However, most modern IDE drives have very similar average seek times (ranging from about 7 to 10 milliseconds). Therefore when choosing a hard drive, seek time is perhaps less important these days than the transfer rate.
Transfer rate
Transfer rate is the amount of data that can be read from or written to the drive in a given amount of time and is usually measured (for sustained transfer rates) in megabytes per second.
Rotational Delay / Latency
This is the average time it takes for the desired sector to appear at the head (assuming that the head is already at the desired track). Clearly, this time is directly related to the rotational speed (spindle speed - see later) of the drive. The average time is the time required for half a rotation. So, for a drive spinning at 7200 revolutions per minute, the average seek time is:
1/2 revolution / 7200 rpm = 0.0000694 minutes = 4.2 ms.
Access Time
This is usually the sum of the seek time and the latency. One needs to be careful when comparing performance figures for hard disks. One drive may boast a seek time of 10ms while another will boast an access time of 12ms. Assuming both drives run at 7200rpm, we can deduce that the seek time of the second drive is actually about 8ms, so it is clearly the quicker of the two.
Performance influencing factors
There are many factors that govern these two performance measurements, but perhaps the three most important ones are:
Spindle speed is very easy to comprehend. It is simply the rotational speed of the spindle, which spins the platters. Typical speeds in today's EIDE/ATA drives are 5400 and 7200 rpm. Obviously, the faster the better. A higher spindle speed produces a higher data transfer rate. This is because more of the drive passes under the drive head in a given amount of time. Note that spindle speed is also indicative of drive quality. The spindle speed itself is governed by the speed of the spindle motor. However, faster speeds require a higher quality platter surface - i.e. flatter.
A small point to note: seek time and access time are not the same thing. Access time actually includes seek time, but also includes: A) command overhead time - the time taken for the mechanics of the drive to respond after a read/write request is received by the circuitry; B) latency - the average amount of time it takes for the correct sector to arrive under the drive head after the head arrives at the correct track (and this is inversely related to spindle speed); C) settle time - the duration between the heads arriving at the correct cylinder, and the heads actually 'settling' into a read/write-ready state. However, these three factors are very small in comparison to the seek time.
Areal density is a little bit more complicated. In simple terms, it is the amount of data present in a given area of a platter (typically measured in the bizarre unit gigabits per square inch). A higher areal density means that there is more data in a given area of platter. A higher areal density improves both transfer rate and seek time. Data transfer rate is improved because more data passes under the drive head as it spins, compared to a drive where there is less data in a given area. Seek time is improved because the tracks are closer together, so the time required to move between them is reduced because less movement for the head on the actuator arm is required.
Lastly, the mechanical design of the actuator assembly is important because these characteristics determine how quickly the heads can be moved over the platters. Obviousy, this isn't a factor that is easily 'measurable' like spindle speed and areal density. The speed of movement of the actuator assembly is more a design and build quality issue.
The next page looks at logical drive performance aspects, such as cluster sizes, fragmentation and slack space.
| Just Too Good Last updated: June, 2006 (DJL)
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