The file system is best visualized as a tree, rooted, as it were, at /. /dev, /usr, and the other directories in the root directory are branches, which may have their own branches, such as /usr/local, and so on.
There are various reasons to house some of these directories on separate file systems. /var contains the directories log/, spool/, and various types of temporary files, and as such, may get filled up. Filling up the root file system is not a good idea, so splitting /var from / is often favorable.
Another common reason to contain certain directory trees on other file systems is if they are to be housed on separate physical disks, or are separate virtual disks, such as Network File System mounts, or CDROM drives.
During the boot process, file systems listed in /etc/fstab are automatically mounted except for the entries
containing noauto. This file contains entries in the
following format:
device /mount-point fstype options dumpfreq passno
An existing device name as explained in Section 19.2.
An existing directory on which to mount the file system.
The file system type to pass to mount(8). The default FreeBSD file system is ufs.
Either rw for read-write file systems, or ro for read-only file systems, followed by any other options
that may be needed. A common option is noauto for
file systems not normally mounted during the boot sequence. Other options are
listed in mount(8).
Used by dump(8) to determine which file systems require dumping. If the field is missing, a value of zero is assumed.
Determines the order in which file systems should be checked. File systems that should be skipped should have their passno set to zero. The root file system needs to be checked before everything else and should have its passno set to one. The other file systems should be set to values greater than one. If more than one file system has the same passno, fsck(8) will attempt to check file systems in parallel if possible.
Refer to fstab(5) for more information on the format of /etc/fstab and its options.
File systems are mounted using mount(8). The most basic syntax is as follows:
This command provides many options which are described in mount(8), The most commonly used options include:
Mount Options
-aMount all the file systems listed in /etc/fstab,
except those marked as “noauto”, excluded by the -t flag, or those that are already mounted.
-dDo everything except for the actual mount system call. This option is useful in
conjunction with the -v flag to determine what mount(8) is
actually trying to do.
-fForce the mount of an unclean file system (dangerous), or the revocation of write access when downgrading a file system's mount status from read-write to read-only.
-rMount the file system read-only. This is identical to using -o ro.
-t fstypeMount the specified file system type or mount only file systems of the given
type, if -a is included. “ufs” is the
default file system type.
-uUpdate mount options on the file system.
-vBe verbose.
-wMount the file system read-write.
The following options can be passed to -o as a
comma-separated list:
Do not allow execution of binaries on this file system. This is also a useful security option.
Do not interpret setuid or setgid flags on the file system. This is also a useful security option.
To unmount a filesystem use umount(8). This
command takes one parameter which can be a mountpoint, device name, -a or -A.
All forms take -f to force unmounting, and -v for verbosity. Be warned that -f is not generally a good idea as it might crash the computer
or damage data on the file system.
To unmount all mounted file systems, or just the file system types listed after
-t, use -a or -A. Note that -A does not attempt
to unmount the root file system.