The smallest unit of organization that FreeBSD uses to find files is the filename. Filenames are case-sensitive, which means that readme.txt and README.TXT are two separate files. FreeBSD does not use the extension of a file to determine whether the file is a program, document, or some other form of data.

Files are stored in directories. A directory may contain no files, or it may contain many hundreds of files. A directory can also contain other directories, allowing you to build up a hierarchy of directories within one another in order to organize data.

Files and directories are referenced by giving the file or directory name, followed by a forward slash, /, followed by any other directory names that are necessary. For example, if the directory foo contains a directory bar which contains the file readme.txt, the full name, or path, to the file is foo/bar/readme.txt. Note that this is different from Windows® which uses \ to separate file and directory names. FreeBSD does not use drive letters, or other drive names in the path. For example, you would not type c:/foo/bar/readme.txt on FreeBSD.

Directories and files are stored in a file system. Each file system contains exactly one directory at the very top level, called the root directory for that file system. This root directory can contain other directories. One file system is designated the root file system or /. Every other file system is mounted under the root file system. No matter how many disks you have on your FreeBSD system, every directory appears to be part of the same disk.

Suppose you have three file systems, called A, B, and C. Each file system has one root directory, which contains two other directories, called A1, A2 (and likewise B1, B2 and C1, C2).

Call A the root file system. If you used ls to view the contents of this directory you would see two subdirectories, A1 and A2. The directory tree looks like this:

A file system must be mounted on to a directory in another file system. When mounting file system B on to the directory A1, the root directory of B replaces A1, and the directories in B appear accordingly:

Any files that are in the B1 or B2 directories can be reached with the path /A1/B1 or /A1/B2 as necessary. Any files that were in /A1 have been temporarily hidden. They will reappear if B is unmounted from A.

If B had been mounted on A2 then the diagram would look like this:

and the paths would be /A2/B1 and /A2/B2 respectively.

File systems can be mounted on top of one another. Continuing the last example, the C file system could be mounted on top of the B1 directory in the B file system, leading to this arrangement:

Or C could be mounted directly on to the A file system, under the A1 directory:

This is similar, although not identical, to a MS-DOS® join.

Typically you create file systems when installing FreeBSD and decide where to mount them, and then never change them unless you add a new disk.

It is entirely possible to have one large root file system, and not need to create any others. There are some drawbacks to this approach, and one advantage.

File systems are contained in partitions. This does not have the same meaning as the common usage of the term partition (for example, MS-DOS® partition), because of FreeBSD's UNIX® heritage. Each partition is identified by a letter from a through to h. Each partition can contain only one file system, which means that file systems are often described by either their typical mount point in the file system hierarchy, or the letter of the partition they are contained in.

FreeBSD also uses disk space for swap space to provide virtual memory. This allows your computer to behave as though it has much more memory than it actually does. When FreeBSD runs out of memory, it moves some of the data that is not currently being used to the swap space, and moves it back in (moving something else out) when it needs it.

Some partitions have certain conventions associated with them.

Each partition-that-contains-a-file-system is stored in what FreeBSD calls a slice. Slice is FreeBSD's term for what the common call partitions, and again, this is because of FreeBSD's UNIX® background. Slices are numbered, starting at 1, through to 4.

Slice numbers follow the device name, prefixed with an s, starting at 1. So “da0s1” is the first slice on the first SCSI drive. There can only be four physical slices on a disk, but you can have logical slices inside physical slices of the appropriate type. These extended slices are numbered starting at 5, so “ad0s5” is the first extended slice on the first IDE disk. These devices are used by file systems that expect to occupy a slice.

Slices, “dangerously dedicated” physical drives, and other drives contain partitions, which are represented as letters from a to h. This letter is appended to the device name, so “da0a” is the a partition on the first da drive, which is “dangerously dedicated”. “ad1s3e” is the fifth partition in the third slice of the second IDE disk drive.

Finally, each disk on the system is identified. A disk name starts with a code that indicates the type of disk, and then a number, indicating which disk it is. Unlike slices, disk numbering starts at 0. Common codes that you will see are listed in Table 4.1, “Disk Device Codes”.

When referring to a partition, include the disk name, s, the slice number, and then the partition letter. Examples are shown in Example 4.1, “Sample Disk, Slice, and Partition Names”.

Example 4.2, “Conceptual Model of a Disk” shows a conceptual model of a disk layout.

When installing FreeBSD, configure the disk slices, create partitions within the slice to be used for FreeBSD, create a file system or swap space in each partition, and decide where each file system will be mounted.

Example 4.2. Conceptual Model of a Disk

This diagram shows FreeBSD's view of the first IDE disk attached to the system. Assume that the disk is 4 GB in size, and contains two 2 GB slices (MS-DOS® partitions). The first slice contains a MS-DOS® disk, C:, and the second slice contains a FreeBSD installation. This example FreeBSD installation has three data partitions, and a swap partition.

The three partitions will each hold a file system. Partition a will be used for the root file system, e for the /var/ directory hierarchy, and f for the /usr/ directory hierarchy.