4.7. Mounting and Unmounting File Systems

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 28.3, “Network File System (NFS)”, or CDROM drives.

4.7.1. The fstab File

During the boot process (Chapter 13, 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-point fstype     options      dumpfreq     passno
device

An existing device name as explained in Table 4.3, “Disk Device Names”.

mount-point

An existing directory on which to mount the file system.

fstype

The file system type to pass to mount(8). The default FreeBSD file system is ufs.

options

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).

dumpfreq

Used by dump(8) to determine which file systems require dumping. If the field is missing, a value of zero is assumed.

passno

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.

4.7.2. Using mount(8)

File systems are mounted using mount(8). The most basic syntax is as follows:

# mount device mountpoint

This command provides many options which are described in mount(8), The most commonly used options include:

Mount Options
-a

Mount all the file systems listed in /etc/fstab, except those marked as noauto, excluded by the -t flag, or those that are already mounted.

-d

Do 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.

-f

Force 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.

-r

Mount the file system read-only. This is identical to using -o ro.

-t fstype

Mount 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.

-u

Update mount options on the file system.

-v

Be verbose.

-w

Mount the file system read-write.

The following options can be passed to -o as a comma-separated list:

nosuid

Do not interpret setuid or setgid flags on the file system. This is also a useful security option.

4.7.3. Using umount(8)

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.

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