5.5. Making the Storage Usable

Once a mass storage device is in place, there is little that it can be used for. True, data can be written to it and read back from it, but without any underlying structure data access is only possible by using sector addresses (either geometrical or logical).
What is needed are methods of making the raw storage a hard drive provides more easily usable. The following sections explore some commonly-used techniques for doing just that.

5.5.1. Partitions/Slices

The first thing that often strikes a system administrator is that the size of a hard drive may be much larger than necessary for the task at hand. As a result, many operating systems have the capability of dividing a hard drive's space into various partitions or slices.
Because they are separate from each other, partitions can have different amounts of space utilized, and that space in no way impacts the space utilized by other partitions. For example, the partition holding the files comprising the operating system is not affected even if the partition holding the users' files becomes full. The operating system still has free space for its own use.
Although it is somewhat simplistic, you can think of partitions as being similar to individual disk drives. In fact, some operating systems actually refer to partitions as "drives". However, this viewpoint is not entirely accurate; therefore, it is important that we look at partitions more closely.

5.5.1.1. Partition Attributes

Partitions are defined by the following attributes:
  • Partition geometry
  • Partition type
  • Partition type field
These attributes are explored in more detail in the following sections.
5.5.1.1.1. Geometry
A partition's geometry refers to its physical placement on a disk drive. The geometry can be specified in terms of starting and ending cylinders, heads, and sectors, although most often partitions start and end on cylinder boundaries. A partition's size is then defined as the amount of storage between the starting and ending cylinders.
5.5.1.1.2. Partition Type
The partition type refers to the partition's relationship with the other partitions on the disk drive. There are three different partition types:
  • Primary partitions
  • Extended partitions
  • Logical partitions
The following sections describe each partition type.
5.5.1.1.2.1. Primary Partitions
Primary partitions are partitions that take up one of the four primary partition slots in the disk drive's partition table.
5.5.1.1.2.2. Extended Partitions
Extended partitions were developed in response to the need for more than four partitions per disk drive. An extended partition can itself contain multiple partitions, greatly extending the number of partitions possible on a single drive. The introduction of extended partitions was driven by the ever-increasing capacities of new disk drives.
5.5.1.1.2.3. Logical Partitions
Logical partitions are those partitions contained within an extended partition; in terms of use they are no different than a non-extended primary partition.
5.5.1.1.3. Partition Type Field
Each partition has a type field that contains a code indicating the partition's anticipated usage. The type field may or may not reflect the computer's operating system. Instead, it may reflect how data is to be stored within the partition. The following section contains more information on this important point.