4.6. Disk Organization

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 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, one 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 are on the FreeBSD system, every directory appears to be part of the same disk.

Consider 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 ls(1) is used to view the contents of this directory, it will show 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:

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.

Benefits of Multiple File Systems
Benefit of a Single File System

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.

PartitionConvention
aNormally contains the root file system.
bNormally contains swap space.
cNormally the same size as the enclosing slice. This allows utilities that need to work on the entire slice, such as a bad block scanner, to work on the c partition. A file system would not normally be created on this partition.
dPartition d used to have a special meaning associated with it, although that is now gone and d may work as any normal partition.

Disks in FreeBSD are divided into slices, referred to in Windows® as partitions, which are numbered from 1 to 4. These are then divided into partitions, which contain file systems, and are labeled using letters.

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 there can be 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 are listed in Table 4.3, “Disk Device Names”.

When referring to a partition, include the disk name, s, the slice number, and then the partition letter. Examples are shown in Example 4.12, “Sample Disk, Slice, and Partition Names”.

Example 4.13, “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.

Table 4.3. Disk Device Names
Drive TypeDrive Device Name
IDE and SATA hard drivesad or ada
SCSI hard drives and USB storage devicesda
IDE and SATA CD-ROM drivesacd or cd
SCSI CD-ROM drivescd
Floppy drivesfd
Assorted non-standard CD-ROM drivesmcd for Mitsumi CD-ROM and scd for Sony CD-ROM devices
SCSI tape drivessa
IDE tape drivesast
RAID drivesExamples include aacd for Adaptec® AdvancedRAID, mlxd and mlyd for Mylex®, amrd for AMI MegaRAID®, idad for Compaq Smart RAID, twed for 3ware® RAID.

Example 4.12. Sample Disk, Slice, and Partition Names
NameMeaning
ad0s1aThe first partition (a) on the first slice (s1) on the first IDE disk (ad0).
da1s2eThe fifth partition (e) on the second slice (s2) on the second SCSI disk (da1).

Example 4.13. 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.


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