8.1. Introduction to Hard Disk Partitions

All hard disks used by Linux and other operating systems on PC hardware are divided into one or more non-overlapping regions called partitions. Sometimes an entire hard disk will be taken up by one partition, but usually your system will have at least two partitions on the primary disk—one for the root filesystem, and one for virtual memory (also known as swap space). As explained in Chapter 5, each partition can be used for either a single filesystem or for virtual memory.

Every partition has a type which identifies the kind of data that it stores. There is a type for Linux filesystems, a type for Linux swap space, a type for Windows filesystems, and many more. Almost every kind of operating system that runs on PC hardware has its own partition type for its own filesystems. When adding new partitions on your system, however, you will very rarely use any types other than those specifically for Linux.

On PC systems, each hard disk can only contain four primary partitions. Because this is often not enough, it is possible for one of those four to be a special extended partition that can contain an unlimited number of logical partitions. If you make use of an extended partition, there is effectively no limit on the number that your hard disk can contain.

Every hard disk is divided into equal-sized cylinders, which represent concentric circles on the surface of the disk. Larger hard disks generally have more cylinders, but due to different drive geometries this is not always the case. Each partition has a starting and ending cylinder and occupies all the space on the disk between them.

Be very careful when changing or reformatting any existing partitions on your system. Because they contain filesystem data, deleting or modifying one could wipe out all your files or make your system unbootable. Webmin tries to prevent this, but it is still possible to do a lot of damage with only a few mouse clicks! Normally you should only need to create or edit partitions when adding a new hard disk to your system.