The hard disk is where all data is stored - the operating system, ancillary programs, and HTML/images/movies/etc for every webpage.

The hard disk is an often overlooked bottleneck in the server architecture. While many people correctly focus on the CPU and memory constraints, they incorrectly only focus on the hard disk size. This is a misunderstanding of how a hard disk operates.

Just like RAM, a hard disk not only varies in size, but also in speed of data accessed. Unlike RAM, hard disk space is cheap - Adding another hard disk or getting a bigger hard disk is not a big expense. What is really important is how fast the hard disk responds.

A hard disk has three main stats - its storage space, its seek time (how long it takes to find data), and its RPM (how 'fast' the hard disk operates). In a server situation, the seek time and RPM become increasingly important.