Many people aren't sure where to begin when choosing a hard drive. They generally understand gigabytes but beyond that it all gets blurry. Access time, RPMs, transfer rate, etc. It's a mystery I'm hoping to shed some light on. I will be using performance data from the amazing storagereview.com to try and explain what really determines the performance level of a hard drive.

When you are looking at the specs of a hard drive there are 4 things that are most notable:

1) The number of gigabytes on the drive. This one is simple. This is simply how much storage is available on the drive.

2) The spindle speed. 5400RPM, 7200RPM, 10000RPM, etc. This is telling you how fast the platters inside the drive spin at. The higher the RPMs, the faster the drive.

3) The cache size. This one is more tricky. You can find drives with 2MB of cache, 8MB of cache and even 16MB of cache. It's not quite as cut and dry as spindle speed for determining performance, however.

4) The drive interface. Is it regular ATA (parallel), Serial ATA or SCSI? Many people are unsure what interface is right for their computer or if one drive is faster than another.

Which of these actually make a difference in hard drive performance? Well, #4 doesn't really make any difference in-of-itself, but each standard brings different features which can affect performance. SATA and SCSI offer Native Command Queuing for instance, where PATA does not. For the most, however, these differences are negliable and don't always mean a lot to home users. #1 can make a difference but mainly only in transfer rate. The higher the number of gigabytes per platter, the tighter the density and the faster the heads can read the data. #3 is a bit trickier. With some drives having 16MB of cache, is this really necessary? A lot of the reason for the higher amount of cache is nothing more than marketing. Given that RAM is so cheap these days, manufacturers can add a buttload for very little money and use this while marketing their drives. I believe storagereview says it best with this quote: "extra cache won't hurt performance; but neither does it greatly improve it."

This leaves us with #2. The good majority of performance differences in drives can be directly related to the spindle speed.

So let's break down exactly what's important when choosing a hard drive. There are 3 key areas:

1) Average seek time -- This is a culmination of many things, but it essentially tells you how fast the heads can get to your data and start reading it. The lower this number (in milliseconds) the better.

2) Transfer rate -- This is how fast the data starts being read of the drive once the heads locate it. The higher the number (in megabytes) the better. Probably the single most important factor when considering a storage drive since the files will be larger and there will be more sequential reading/writing.

3) I/Os per second -- This is how many operations (reading and writing data) your hard drive can do in one second. The higher the number the better. Probably the single most important factor when you're looking for a system/boot drive since Windows does a lot of reading/writing.

So let's compare the three modern drives of different spindle speeds. Each of these is currently (in June 2005) the performance leader at their spindle speed.

1) The Maxtor Maxline III, a 300GB, 7200RPM SATA drive with 16MB of cache.

2) The Western Digital Raptor WD740GD, a 74GB, 10000RPM SATA drive with 8MB of cache.

3) The Maxtor Atlas 15K II, a 147GB, 15000RPM SCSI drive with 8MB of cache.

The performance specs for each are as follows:

Average access time (average seek time + latency):
Maxline: 13.5ms -- Raptor: 7.8ms -- Atlas: 5.5ms

Transfer rate (sequential across the drive, from fastest to slowest):
Maxline: 67.7MB/s -> 37.2MB/s -- Raptor: 71.8MB/s -> 53.8MB/s -- Atlas: 97.4MB/s -> 74.4MB/s

I/Os per second in key categories:
Office:: Maxline: 472 -- Raptor: 606 -- Atlas: 725
High-End:: Maxline: 472 -- Raptor: 585 -- Atlas: 637
Bootup:: Maxline: 490 -- Raptor: 610 -- Atlas: 647
Gaming:: Maxline: 589 -- Raptor: 749 -- Atlas: 899

Idle noise of drive (in dB/A @ 18mm):
Maxline: 41.2dB/A -- Raptor: 42.3dB/A -- Atlas: 46.5dB/A

Net Drive Temperature (in degrees celsius):
Maxline: 19.5° -- Raptor: 22.3° -- Atlas: 30.6°

(For more information regarding I/Os per second, click here.)

So, using all the data above what are we able to determine?

Across the board we can see that the faster the spindle speed, the higher the performance. This is why comparing 7200RPM drives to 10000RPM drives is like comparing apples to oranges. They are in completely different classes. As such, when choosing a drive you really have to ask more personal questions.

For instance, since you can't currently get a 15000RPM ATA drive, is the extra performance worth it to get a SCSI controller and pay exorbitant prices for the SCSI hard drive? For most of us, the answer is no. The speed increases over the 10000RPM SATA drive don't seem worth it for a home user.

So if you want to stick with a SATA drive, should you pay more (much more) for a 10000RPM drive over a 7200RPM drive? You will likely pay over 3x as much per gigabyte to get a 10000RPM drive. For some of us that's well worth the performance increase you can see above.

The best thing to do is determine how much you're willing to pay for a performance increase and then decide which drive speed is right for you. After that you need to look into all the drives in that speed and see which one performs the best.