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, described in Section 29.3, “Network File System (NFS)”, or CDROM drives.
During the boot process (Chapter 12, The FreeBSD Booting 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-pointfstypeoptionsdumpfreqpassno
deviceAn existing device name as explained in Table 3.3, “Disk Device Names”.
mount-pointAn existing directory on which to mount the file system.
fstypeThe file system type to pass to mount(8). The
default FreeBSD file system is
ufs.
optionsEither 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).
dumpfreqUsed by dump(8) to determine which file systems require dumping. If the field is missing, a value of zero is assumed.
passnoDetermines 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:
#mountdevicemountpoint
This command provides many options which are described in mount(8), The most commonly used options include:
-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 interpret setuid or setgid flags on the file system. This is also a useful security option.
To unmount a file system 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.
All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.