Hard Disk Vocabulary

by Jimmy Martin

There are several important technical terms to describe hard disk performance. They sound complicated at first, but when you know how a hard disk drive works, they are actually very simple. Knowing what these terms mean can help you to choose a hard disk drive, or understand whether a web hosting company is giving you a good deal on a server.

Important Technical Terms

Capacity - How much data can be stored on the drive. This is measured in Gigabytes (GB). Note that hard disk manufacturers use what is known as a decimal Gigabyte, which is 10 to the power 9 or 1,000,000,000 bytes. Computers use what is known as a binary Gigabyte, which is 2 to the power 30 or 1,073,741,824 bytes. Therefore the hard disk capacity in GB claimed by the manufacturer is smaller than the number your computer will report. This has been standard practise for many years, but it can catch out beginners.

Spindle Speed - This is simply how fast the platters (metal disks that store the data) rotate. It is usually between 5,400 and 15,000 rpm. In a low- to mid-range web server you should be looking for a 7,200 rpm drive. A high end server would have 10,000 or 15,000 rpm drives. The faster the spindle speed, the faster the drive can read and write data.

Seek time - A measure of how long it takes for a hard disk drive to move its heads to the correct part of the platters to find the data that it is looking for. This is measured in milliseconds (ms). This will be somewhere around 10 ms, give or take 5 ms. The lower the figure, the faster the drive finds the data.

Cache - The drive stores some data in Random Access Memory known as the cache buffer. It is much faster to access the buffer than it is to find data on the drive. The drive controller keeps data that you have recently used in the cache in case you need it again, and it also reads data from the disk that it thinks you might ask for next. This makes it faster to access data, so long as the data you asked for is in the cache. Otherwise the drive will have to find it on disk. Cache memory is especially important for accessing small, frequently-used files. Cache sizes start from around 8 Megabytes, the more the better.

Sustained Transfer Rate - How fast the drive can transfer large files that do not fit into the cache buffer, or small files that were not in the cache. The greater the rate, the better, especially if most of your data is stored in large files.

Interface - This is the connection between the drive and the computer's processor. Older servers use PATA, newer ones use SATA. High-end, expensive servers use SCSI, and very high-end servers use Fibre Channel.

Next we will look at the different hard disk interfaces used on web servers.