Written by James Newcomb
Tuesday May 5, 2009
Page 4 of 5

All Computers are not created equal - Hard Drives

Hard drive specs are a bit easier to understand because they are a physical device with moving parts, and as such people can relate better to how they work. The important specs to know when it comes to performance are average seek time, RPM and how the hard drive interfaces to the computer.

Average seek time is the time it takes for the read/write head to move from one random track on the drive to another track. RPMs are revolutions per minute, which is how fast the platters inside the hard drive are spinning. The RPM of the drive introduces a kind of latency which is added to the seek time: rotational latency. The hard drive interface is what sets the "speed limit" for how fast the data gets transferred to and from the rest of the computer hardware.

To explain this better I'll use an example: a 7200 RPM drive with an average seek time of 8.9 milliseconds (ms)

On a 7200 RPM drive, the platters are spinning at 120 revolutions per second, which is 8.3 ms per rotation. Since data is scattered all across the drive, it could take anywhere from 0 to 8.3 ms to get to the beginning of a data track, therefore the average rotational latency would be 4.15 ms. Average rotational latency plus average seek time gives us our average access time, which would be 13.05 ms for the hard drive to find the file we want

If we took a 5400 RPM drive with the same average seek time of 8.9 ms, we would have an average access time of 14.45 ms just because of the added rotational latency.

While a 1.4 ms difference doesn't sound like a long time, the computer can potentially request thousands of files to be accessed in a short period of time, like when you're starting your computer. Adding 1.4 ms to EACH file it's trying to read can slow your computer down quite a bit!

With a fast interface such as Serial Advanced Technology Attachment (SATA) 1.5 Gigabits per second (Gb/s) most hard drives can run at full, or near-full speed capacity without being bottlenecked by the interface. SATA 3.0 Gb/s interfaces are well beyond what hard drives by today's standards are capable of transferring, but the 1.5 Gb/s interface can be saturated, and bottleneck data throughput on some extremely high performance hard drives.

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