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 (unless they are listed with the
noauto option).
The /etc/fstab file contains a list of lines of the following format:
device /mount-point fstype options dumpfreq passno
A device name (which should exist), as explained in Section 12.2.
A directory (which should exist), on which to mount the file system.
The file system type to pass to mount(8). The default DragonFly 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 the mount(8) manual page.
This is used by dump(8) to determine which file systems require dumping. If the field is missing, a value of zero is assumed.
This 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 (which needs to be checked before everything else) should have its passno set to one, and other file systems' passno should be set to values greater than one. If more than one file systems have the same passno then fsck(8) will attempt to check file systems in parallel if possible.
Consult the fstab(5) manual page for more information on the format of the /etc/fstab file and the options it contains.
The mount(8) command is what is ultimately used to mount file systems.
In its most basic form, you use:
There are plenty of options, as mentioned in the mount(8) manual page, but the most common are:
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 forces 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 the rdonly argument to the
-o option.
-t
fstypeMount the given file system as the given file system
type, or mount only file systems of the given type, if
given the -a option.
``ufs'' is the default file system type.
-uUpdate mount options on the file system.
-vBe verbose.
-wMount the file system read-write.
The -o option takes a comma-separated list of
the options, including the following:
Do not interpret special devices on the file system. This is a useful security option.
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.
The umount(8) command takes, as a parameter, one of a
mountpoint, a device name, or the -a or
-A option.
All forms take -f to force unmounting,
and -v for verbosity. Be warned that
-f is not generally a good idea. Forcibly
unmounting file systems might crash the computer or damage data
on the file system.
-a and -A are used to
unmount all mounted file systems, possibly modified by the
file system types listed after -t.
-A, however, does not attempt to unmount the
root file system.
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