Linux mount and umount

Updated: 11/06/2021 by Computer Hope
mount command

The mount command mounts a storage device or filesystem, making it accessible and attaching it to an existing directory structure.

The umount command "unmounts" a mounted filesystem, informing the system to complete any pending read or write operations, and safely detaching it.


mount [-lhV]
mount -a [-fFnrsvw] [-t vfstype] [-O optlist]
mount [-fnrsvw] [-o option[,option]...] device|dir
mount [-fnrsvw] [-t vfstype] [-o options] device|dir

umount syntax

umount [-hV]
umount -a [-dflnrv] [-t vfstype] [-O options]
umount [-dflnrv] {dir|device}...

Description: mount

All files accessible in Unix, or a Unix-style system such as Linux, are arranged in one big tree: the file hierarchy, rooted at /. These files can be spread out over several devices. The mount command attaches a filesystem, on some device or other, to the file tree. Conversely, the umount command detaches it again.

The standard form of the mount command is:

mount -t type device dir

This tells the kernel to attach the filesystem found on device (which is of type type) at the directory dir. The previous contents (if any), owner, and mode of dir become invisible, and as long as this filesystem remains mounted, the pathname dir refers to the root of the filesystem on device.

If only directory or device is given, for example:

mount /dir

then mount looks for a corresponding mountpoint (and then, if not found, for a corresponding device) entry in the /etc/fstab file, and attempts to mount it.

Description: umount

The umount command detaches the specified file system(s) from the file hierarchy. A file system is specified by giving the directory where it was mounted. Giving the special device on which the file system lives may also work, but is an obsolete method, mainly because it fails in case this device was mounted on more than one directory.

Note that a file system cannot be unmounted when it is busy - for example, when there are open files on it, or when some process has its working directory there, or when a swap file is in use. The offending process could even be umount itself - for example, umount opens the common C libraries, which in turn may open (for example) locale files, which, if they are on the device in question, prevents it from being unmounted. A "lazy" unmount (see -l in the umount options section, below) attempts to unmount even if this conflict arises.

Listing Mounts And Getting Help

Three forms of the mount command do not actually mount anything:

mount -h

prints a help message, and exits;

mount -V

prints mount's version information, and exits;

mount [-l] [-t type]

lists all mounted filesystems (of type type). The option -l adds labels to this listing.

Device Indication

Most devices are indicated by a file name (of a block special device) like /dev/sda1, but there are other possibilities. For example, in the case of an NFS mount, the device name may look like hostname:/dir. It is possible to indicate a block special device using its volume LABEL or UUID (see the -L and -U options below).

The recommended setup is to use "LABEL=<label>" or "UUID=<uuid>" tags rather than "/dev/disk/by-{label,uuid}" udev symlinks in the /etc/fstab file. The tags are more readable, robust and portable. Internally, mount command uses udev symlinks, so using symlinks in /etc/fstab has no advantage over "LABEL=/UUID=".

Note that mount uses UUIDs as strings. The UUIDs from command line or fstab are not converted to internal binary representation. The string representation of the UUID should be based on lowercase characters.

The proc filesystem is not associated with a special device, and when mounting it, an arbitrary keyword such as proc can be used instead of a device specification. The customary choice none is less fortunate: the error message "none busy" from umount can be confusing since something is indeed busy.

The Files /etc/fstab, /etc/mtab And /proc/mounts

The file /etc/fstab may contain lines describing what devices are usually mounted where, using which options.

The command

mount -a [-t type] [-O optlist]

causes all filesystems mentioned in fstab (of the proper type and/or having or not having the proper options) to be mounted as indicated, except for those whose line contains the noauto keyword. This command is often included in a boot script. Adding the -F option makes mount fork, so that the filesystems are mounted simultaneously.

When mounting a filesystem mentioned in fstab or mtab, it suffices to give only the device, or only the mount point. The programs mount and umount maintain a list of currently mounted filesystems in the file /etc/mtab. If no arguments are given to mount, this list is printed.

The mount program does not read the /etc/fstab file if both device (or LABEL/UUID) and dir are specified. For example:

mount /dev/foo /dir

To override mount options from /etc/fstab, you have to use:

mount device|dir -o options

and then the mount options from command line is appended to the list of options from /etc/fstab. If there are duplicate options, the usual behavior is that the one occurring last in the command is used.

When the proc filesystem is mounted (at /proc, for instance), the files /etc/mtab and /proc/mounts have very similar contents. The former has somewhat more information, such as the mount options used, but is not necessarily up-to-date (compare with the -n option below). It is possible to replace /etc/mtab by a symbolic link to /proc/mounts. When you have very large numbers of mounts things are much faster with the symlink, but some information is lost that way; in particular, the "user" option fails.

Non-Superuser Mounts

Normally, only the superuser can mount filesystems. However, when fstab contains the user option on a line, anybody can mount the corresponding system.

Thus, given an fstab line

/dev/cdrom /cd iso9660 ro,user,noauto,unhide

any user can mount the iso9660 filesystem found on a CD-ROM, using the command

mount /dev/cdrom


mount /cd

By default, only the user that mounted a filesystem can unmount it. If you'd like to allow any user to be able to unmount a user-mount filesystem, use "users" instead of "user" in the fstab line. The "owner" option is similar to the "user" option, with the restriction that the user must be the owner of the special file. This may be useful, for example, for /dev/fd if a login script makes the console user owner of this device. The "group" option is similar, with the restriction that the user must be member of the special file's owning group.

Bind Mounts

Since Linux 2.4.0, it is possible to remount part of the file hierarchy somewhere else. The call is as follows:

mount --bind olddir newdir

or, using the short option:

mount -B olddir newdir

or as an fstab entry:

/olddir /newdir none bind

After this call the same contents is accessible in two places. One can also remount a single file (on a single file). It's also possible to use the bind mount to create a mountpoint from a regular directory, for example:

mount --bind foo foo

The bind mount call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place using

mount --rbind olddir newdir

or, using the short option:

mount -R olddir newdir

Note that the filesystem mount options remains the same as those on the original mount point, and cannot be changed by passing the -o option with --bind/--rbind. The mount options can be changed by a separate remount command, for example:

mount --bind olddir newdir
mount -o remount,ro newdir

Note that behavior of the remount operation depends on the /etc/mtab file. The first command stores the 'bind' flag to the /etc/mtab file and the second command reads the flag from the file. If you have a system without the /etc/mtab file or if you explicitly define source and target for the remount command (then mount does not read /etc/mtab), then you have to use bind flag (or option) for the remount command too. For example:

mount --bind olddir newdir
mount -o remount,ro,bind olddir newdir

The Move Operation

Since Linux 2.5.1, it is possible to atomically move a mounted tree to another place. The call is:

mount --move olddir newdir

or shortoption

mount -M olddir newdir

This causes the contents which previously appeared under olddir to be accessed under newdir. The physical location of the files is not changed. Note that the olddir has to be a mountpoint.

Shared Subtree Options

Since Linux 2.6.15 it is possible to mark a mount and its submounts as shared, private, secondary or unbindable. A shared mount provides ability to create mirrors of that mount such that mounts and umounts within any of the mirrors propagate to the other mirror. A secondary mount receives propagation from its primary, but any not vice-versa. A private mount carries no propagation abilities. A unbindable mount is a private mount which cannot be cloned through a bind operation. Detailed semantics is documented in Documentation/filesystems/sharedsubtree.txt file in the kernel source tree.

mount --make-shared mountpoint
mount --make-slave mountpoint
mount --make-private mountpoint
mount --make-unbindable mountpoint

The following commands allows one to recursively change the type of all the mounts under a given mountpoint.

mount --make-rshared mountpoint
mount --make-rslave mountpoint
mount --make-rprivate mountpoint
mount --make-runbindable mountpoint

mount: Command Line Options

The full set of mount options used by an invocation of mount is determined by first extracting the mount options for the filesystem from the fstab table, then applying any options specified by the -o argument, and finally applying a -r or -w option, when present.

Options are as follows:

-V, --version Display version information, and exit.
-h, --help Display a help message, and exit.
-v, --verbose Operate verbosely.
-a, --all Mount all filesystems (of the given types) mentioned in fstab.
-F, --fork (Used in conjunction with -a): fork off a new incarnation of mount for each device. This does the mounts on different devices or different NFS servers in parallel. This has the advantage that it is faster; also, NFS timeouts occur in parallel. A disadvantage is that the mounts are done in undefined order. Thus, you cannot use this option if you want to mount both /usr and /usr/spool since /usr must be mounted first.
-f, --fake Causes everything to be done except for the actual system call; in effect, this "fakes" the mounting of the filesystem. This option is useful in conjunction with the -v flag to determine what the mount command is trying to do. It can also be used to add entries for devices that were mounted earlier with the -n option. The -f option checks for existing record in /etc/mtab and fails when the record already exists (with regular non-fake mount operations, this check is done by the kernel).
-i, --internal-only Don't call the /sbin/mount.filesystem helper, even if it exists.
-l Add labels to the mount output. Mount must have permission to read the disk device (e.g., be suid root) for this to work. One can set such a label for ext2, ext3 or ext4 using the e2label utility, or for XFS using xfs_admin, or for reiserfs using reiserfstune.
-n, --no-mtab Mount without writing in /etc/mtab. This is necessary, for example, when /etc is on a read-only filesystem.
--no-canonicalize Don't canonicalize (standardize) path names. The mount command canonicalizes all paths (from command line or fstab) and stores canonicalized paths to the /etc/mtab file. This option can is used with the -f flag for already canonicalized absolute paths.
-p, --pass-fd num In case of a loop mount with encryption, read the passphrase from file descriptor num instead of from the terminal.
-s Tolerate sloppy mount options rather than failing. This option ignores mount options not supported by a filesystem type. Not all filesystems support this option. This option exists for support of the Linux autofs-based automounter.
-r, --read-only Mount the filesystem read-only. A synonym is -o ro.

Note that, depending on the filesystem type, state and kernel behavior, the system may still write to the device. For example, ext3 or ext4 replays its journal if the filesystem is "dirty" (if there is unfinished business, such as data not yet written). To prevent this kind of write access, you may want to mount ext3 or ext4 filesystems with "ro,noload" mount options or set the block device to read-only mode, which can be achieved with the command blockdev.
-w, --rw Mount the filesystem as read/write. This is the default. A synonym is -o rw.
-L label Mount the partition withthe specified label.
-U uuid Mount the partition with the specified UUID uuid. These two options require the file /proc/partitions (present since Linux 2.1.116) to exist.
-t, --types vfstype The argument following the -t indicates the filesystem type. The filesystem types that are currently supported include: Note that coherent, sysv and xenix are equivalent and that xenix and coherent are removed at some point in the future — use sysv instead. Since kernel version 2.1.21, the types ext and xiafs do not exist anymore. Earlier, usbfs was known as usbdevfs. Note also that the complete list of all supported filesystems depends upon your kernel.

The programs mount and umount support filesystem subtypes. The subtype is defined by '.subtype' suffix. For example, 'fuse.sshfs'. It's recommended to use subtype notation rather than add any prefix to the mount source (for example, '' is deprecated).

For most types all the mount program has to do is issue a simple mount( ) system call, and no detailed knowledge of the filesystem type is required. For a few types however (like nfs, nfs4, cifs, smbfs, ncpfs) ad hoc code is necessary. The nfs, nfs4, cifs, smbfs, and ncpfs filesystems have a separate mount program. To make it possible to treat all types in a uniform way, mount executes the program /sbin/mount.TYPE (if that exists) when called with type TYPE. Since various versions of the smbmount program have different calling conventions, /sbin/mount.smbfs may have to be a shell script that sets up the desired call.

If no -t option is given, or if the auto type is specified, mount tries to guess the desired type. mount uses the blkid library for guessing the filesystem type; if that does not turn up anything that looks familiar, mount tries to read the file /etc/filesystems, or, if that does not exist, /proc/filesystems. All the filesystem types listed there are tried, except for those that are labeled "nodev" (e.g., devpts, proc and nfs). If /etc/filesystems ends in a line with a single * only, mount reads /proc/filesystems afterwards.

The auto type may be useful for user-mounted floppies. Creating a file /etc/filesystems can be useful to change the probe order (e.g., to try vfat before msdos or ext3 before ext2) or if you use a kernel module autoloader.

" More than one type may be specified in a comma separated list. The list of filesystem types can be prefixed with no to specify the filesystem types on which no action should be taken. This can be meaningful with the -a option. For example, the command:

mount -a -t nomsdos,ext
mounts all filesystems except those of type msdos and ext.
-O, --test-opts opts Used in conjunction with -a, to limit the set of filesystems that the -a is applied. Like -t in this regard except that it is useless except in the context of -a. For example, the command:

mount -a -O no_netdev
mounts all filesystems except those which have the option _netdev specified in the options field in the /etc/fstab file.

It is different from -t in that each option is matched exactly; a leading no at the beginning of one option does not negate the rest.

The -t and -O options are cumulative in effect; that is, the command

mount -a -t ext2 -O _netdev
mounts all ext2 filesystems with the _netdev option, not all filesystems that are either ext2 or have the _netdev option specified.
-o, --options opts Options are specified with a -o flag followed by a comma separated string of options. For example:

mount LABEL=mydisk -o noatime,nouser
For more details, see Filesystem Independent mount and Filesystem Specific mount sections.
-B, --bind Remount a subtree somewhere else (so that its contents are available in both places).
-R, --rbind Remount a subtree and all possible submounts somewhere else (so that its contents are available in both places).
-M, --move Move a subtree to some other place.


Some of these options are only useful when they appear in the /etc/fstab file.

Some of these options could be enabled or disabled by default in the system kernel. To check the current setting see the options in /proc/mounts.

The following options apply to any filesystem that is being mounted (but not every filesystem actually honors them - e.g., the sync option today has effect only for ext2, ext3, fat, vfat and ufs):

async All I/O to the filesystem should be done asynchronously. See also the sync option.
atime Do not use noatime feature, then the inode access time is controlled by kernel defaults. See also the description for strictatime and reatime mount options.
noatime Do not update inode access times on this filesystem (e.g., for faster access on the news spool to speed up news servers).
auto Can be mounted with the -a option.
noauto Can only be mounted explicitly (i.e., the -a option won't cause the filesystem to be mounted).
context=context, fscontext=context, defcontext=context, rootcontext=context The context= option is useful when mounting filesystems that do not support extended attributes, such as a floppy or hard disk formatted with VFAT, or systems that are not normally running under SELinux, such as an ext3 formatted disk from a non-SELinux workstation. You can also use context= on filesystems you do not trust, such as a floppy. It also helps in compatibility with xattr-supporting filesystems on earlier 2.4.<x> kernel versions. Even where xattrs are supported, you can save time not having to label every file by assigning the entire disk one security context.

A commonly used option for removable media is context=system_u:object_r:removable_t.

Two other options are fscontext= and defcontext=, which are mutually exclusive of the context option. This means you can use fscontext and defcontext together, but neither can be used with context.

The fscontext= option works for all filesystems, regardless of their xattr support. The fscontext option sets the overarching filesystem label to a specific security context. This filesystem label is separate from the individual labels on the files. It represents the entire filesystem for certain kinds of permission checks, such as during mount or file creation. Individual file labels are still obtained from the xattrs on the files themselves. The context option actually sets the aggregate context that fscontext provides, in addition to supplying the same label for individual files.

You can set the default security context for unlabeled files using defcontext= option. This overrides the value set for unlabeled files in the policy and requires a filesystem that supports xattr labeling.

The rootcontext= option allows you to explicitly label the root inode of an FS being mounted before that FS or inode becomes visible to userspace. This was found to be useful for things like Red Hat stateless linux.

Note that kernel rejects any remount request that includes the context option even if unchanged from the current context.
defaults Use default options: rw, suid, dev, exec, auto, nouser, and async.
dev Interpret character or block special devices on the filesystem.
nodev Do not interpret character or block special devices on the file system.
diratime Update directory inode access times on this filesystem. This is the default.
nodiratime Do not update directory inode access times on this filesystem.
dirsync All directory updates in the filesystem should be done synchronously. This affects the following system calls: creat, link, unlink, symlink, mkdir, rmdir, mknod and rename.
exec Permit execution of binaries.
noexec Do not allow direct execution of any binaries on the mounted filesystem. Until recently it was possible to run binaries using a command like /lib/ld*.so /mnt/binary. This trick fails since Linux 2.4.25 / 2.6.0.
group Allow an ordinary (i.e., non-root) user to mount the filesystem if one of his groups matches the group of the device. This option implies the options nosuid and nodev (unless overridden by subsequent options, as in the option line group,dev,suid).
encryption Specifies an encryption algorithm to use. Used in conjunction with the loop option.
keybits Specifies the key size to use for an encryption algorithm. Used in conjunction with the loop and encryption options.
iversion Every time the inode is modified, the i_version field is incremented.
noiversion Do not increment the i_version inode field.
mand Allow mandatory locks on this filesystem.
nomand Do not allow mandatory locks on this filesystem.
_netdev The filesystem resides on a device that requires network access (used to prevent the system from attempting to mount these filesystems until the network is enabled on the system).
nofail Do not report errors for this device if it does not exist.
relatime Update inode access times relative to modify or change time. Access time is only updated if the previous access time was earlier than the current modify or change time. Similar to noatime, but doesn't breakapplications that need to know if a file is read since the last time it was modified.

Since Linux 2.6.30, the kernel defaults to the behavior provided by this option (unless noatime was specified), and the strictatime option is required to obtain traditional semantics. Also since Linux 2.6.30, the file's last access time is always updated if it's more than 1 day old.
norelatime Do not use relatime feature. See also the strictatime mount option.
strictatime Allows to explicitly requesting full atime updates. This makes it possible for kernel to defaults to relatime or noatime, but still allow userspace to override it. For more details about the default system mount options see /proc/mounts.
nostrictatime Use the kernel's default behavior for inode access time updates.
suid Allow set-user-identifier or set-group-identifier bits to take effect.
nosuid Do not allow set-user-identifier or set-group-identifier bits to take effect. This seems safe, but is in fact rather unsafe if you have suidperl installed.
silent Turn on the silent flag.
loud Turn off the silent flag.
owner Allow an ordinary (i.e., non-root) user to mount the filesystem if he is the owner of the device. This option implies the options nosuid and nodev (unless overridden by subsequent options, as in the option line owner,dev,suid).
remount Attempt to remount an already-mounted filesystem. This is commonly used to change the mount flags for a filesystem, especially to make a readonly filesystem writable. It does not change device or mount point.

The remount functionality follows the standard way how the mount command works with options from fstab. It means the mount command doesn't read fstab (or mtab) only when a device and dir are fully specified.

mount -o remount,rw /dev/foo /dir
After this call all old mount options are replaced and arbitrary stuff from fstab is ignored, except the loop= option that is internally generated and maintained by the mount command.

mount -o remount,rw /dir
After this call mount reads fstab (or mtab) and merges these options with options from command line ( -o ).
ro Mount the filesystem read-only.
rw Mount the filesystem read-write.
sync All I/O to the filesystem should be done synchronously. In case of media with limited number of write cycles (e.g., some flash drives) sync may cause life-cycle shortening.
user Allow an ordinary user to mount the filesystem. The name of the mounting user is written to mtab so that he can unmount the filesystem again. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line user,exec,dev,suid).
nouser Forbid an ordinary (i.e., non-root) user to mount the filesystem. This is the default.
users Allow every user to mount and unmount the filesystem. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line users,exec,dev,suid).


The following options apply only to certain filesystems. Here, they are sorted by filesystem. They all follow the -o flag.

What options are supported depends on the running kernel. More info is found in the kernel source subdirectory Documentation/filesystems.

Filesystem-Specific mount Options: adfs

Advanced Disc Filing System ("adfs") is specific to certain RISC operating systems, such as those on the Acorn MOS line of computers.

Mount Options for adfs:

uid=value, gid=value Set the owner and group of the files in the filesystem (default: uid=gid=0).
ownmask=value, othmask=value Set the permission mask for ADFS 'owner' permissions and 'other' permissions, respectively (default: 0700 and 0077, respectively). See also /usr/src/linux/Documentation/filesystems/adfs.txt.

Filesystem-Specific mount Options: affs

Amiga Fast File System ("affs") is a filesystem used on the Amiga personal computer.

Mount options for affs:

uid=value, gid=value Set the owner and group of the root of the filesystem (default: uid=gid=0, but with option uid or gid without specified value, the uid and gid of the current process are taken).
setuid=value, setgid=value Set the owner and group of all files.
mode=value Set the mode of all files to value & 0777 disregarding the original permissions. Add search permission to directories that have read permission. The value is given in octal.
protect Do not allow any changes to the protection bits on the filesystem.
usemp Set uid and gid of the root of the filesystem to the uid and gid of the mount point upon the first sync or umount, and then clear this option.
verbose Print an informational message for each successful mount.
prefix=string Prefix used before volume name, when following a link.
volume=string Prefix (of length at most 30) used before '/' when following a symbolic link.
reserved=value (Default: 2.) Number of unused blocks at the start of the device.
root=value Give explicitly the location of the root block.
bs=value Give blocksize. Allowed values are 512, 1024, 2048, 4096.
grpquota|noquota|quota|usrquota These options are accepted but ignored. (However, quota utilities may react to such strings in /etc/fstab.

Filesystem-Specific mount Options: devpts

The devpts filesystem is a pseudo-filesystem, traditionally mounted on /dev/pts. To acquire a pseudo terminal, a process opens /dev/ptmx; the number of the pseudo terminal is then made available to the process and the pseudo terminal secondary can be accessed as /dev/pts/<number>.

Mount options for devpts:

uid=value, gid=value This sets the owner or the group of newly created PTYs to the specified values. When nothing is specified, they are set to the UID and GID of the creating process. For example, if there is a tty group with GID 5, then gid=5 causes newly created PTYs to belong to the tty group.
mode=value Set the mode of newly created PTYs to the specified value. The default is 0600. A value of mode=620 and gid=5 makes "mesg y" the default on newly created PTYs.
newinstance Create a private instance of devpts filesystem, such that indices of ptys allocated in this new instance are independent of indices created in other instances of devpts.

All mounts of devpts without this newinstance option share the same set of pty indices (i.e legacy mode). Each mount of devpts with the newinstance option has a private set of pty indices.

This option is mainly used to support containers in the linux kernel. It is implemented in linux kernel versions starting with 2.6.29. Further, this mount option is valid only if CONFIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configuration.

To use this option effectively, /dev/ptmx must be a symbolic link to pts/ptmx. See Documentation/filesystems/devpts.txt in the linux kernel source tree for details.
ptmxmode=value Set the mode for the new ptmx device node in the devpts filesystem.

With the support for multiple instances of devpts (see newinstance option above), each instance has a private ptmx node in the root of the devpts filesystem (often /dev/pts/ptmx).

For compatibility with older versions of the kernel, the default mode of the new ptmx node is 0000. ptmxmode=value specifies a more useful mode for the ptmx node and is highly recommended when the newinstance option is specified.

This option is only implemented in linux kernel versions starting with 2.6.29. Further this option is valid only if CONFIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configuration.

Filesystem-Specific mount Options: ext2

ext2, the "second extended file system", was once the default filesystem of many distributions of Linux, including Debian and Red Hat Linux. It has since largely been replaced by ext3 (see below), but is still widely used, especially on flash-based solid-state media.

Mount options for ext2:

acl|noacl Support POSIX Access Control Lists (or do not).
bsddf|minixdf Set the behavior for the statfs system call. The minixdf behavior is to return in the f_blocks field the total number of blocks of the filesystem, while the bsddf behavior (which is the default) is to subtract the overhead blocks used by the ext2 filesystem and not available for file storage. Thus:

mount /k -o minixdf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on /dev/sda6 2630655 86954 2412169 3% /k
% mount /k -o bsddf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on /dev/sda6 2543714 13 2412169 0% /k
Note that this example shows that one can add command line options to the options given in /etc/fstab.
check={none|nocheck} No checking is done at mount time. This is the default. This is fast. It is wise to invoke e2fsck every now and then, e.g., at boot time.
debug Print debugging info upon each (re)mount.
errors={continue|remount-ro|panic} Define the behavior when an error is encountered. Either ignore errors and mark the filesystem erroneous and continue, or remount the filesystem read-only, or panic and halt the system. The default is set in the filesystem superblock, and can be changed using tune2fs.
grpid|bsdgroups, nogrpid|sysvgroups These options define what group id a newly created file gets. When grpid is set, it takes the group id of the directory where it is created; otherwise (the default) it takes the fsgid of the current process, unless the directory has the setgid bit set, where case it takes the gid from the parent directory, and also gets the setgid bit set if it's a directory itself.
grpquota|noquota|quota|usrquota These options are accepted but ignored.
nouid32 Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only store and expect 16-bit values.
oldalloc, orlov Use old allocator or Orlov allocator for new inodes. Orlov is default.
resgid=n, resuid=n The ext2 filesystem reserves a certain percentage of the available space (by default 5%). These options determine who can use the reserved blocks. Roughly whoever has the specified uid, or belongs to the specified group.
sb=n Instead of block 1, use block n as superblock. This could be useful when the filesystem is damaged. (Earlier, copies of the superblock would be made every 8192 blocks: in block 1, 8193, 16385, etc. (and one got thousands of copies on a big filesystem). Since version 1.08, mke2fs has a -s (sparse superblock) option to reduce the number of backup superblocks, and since version 1.15 this is the default. Note that this may mean that ext2 filesystems created by a recent mke2fs cannot be mounted r/w under Linux 2.0.*. The block number here uses 1k units. Thus, if you want to use logical block 32768 on a filesystem with 4k blocks, use "sb=131072".
user_xattr|nouser_xattr Support "user." extended attributes (or do not).

Filesystem-Specific mount Options: ext3

The ext3 filesystem is a version of ext2 that is enhanced with journalling features. It supports all the ext2 mount options listed above, and the following options.

Mount Options for ext3:

journal=update Update the ext3 filesystem's journal to the current format.
journal=inum When a journal already exists, this option is ignored. Otherwise, it specifies the number of the inode which represents the ext3 filesystem's journal file; ext3 creates a new journal, overwriting the old contents of the file whose inode number is inum.
journal_dev=devnum When the external journal device's major/minor numbers have changed, this option allows the user to specify the new journal location. The journal device is identified through its new major/minor numbers encoded in devnum.
norecovery/noload Don't load the journal on mounting. Note that if the filesystem was not unmounted cleanly, skipping the journal replay leads to the filesystem containing inconsistencies that lead to any number of problems.

Specifies the journalling mode for file data. Metadata is always journaled. To use modes other than ordered on the root filesystem, pass the mode to the kernel as boot parameter, e.g., rootflags=data=journal.

journal All data is committed into the journal before being written into the main filesystem.
ordered This is the default mode. All data is forced directly out to the main file system prior to its metadata being committed to the journal.
writeback Data ordering is not preserved: data may be written into the main filesystem after its metadata is committed to the journal. This is rumoured to be the highest-throughput option. It guarantees internal filesystem integrity, however it can allow old data to appear in files after a crash and journal recovery.
barrier=0 / barrier=1 This enables/disables barriers. barrier=0 disables it, barrier=1 enables it. Write barriers enforce proper on-disk ordering of journal commits, making volatile disk write caches safe to use, at some performance penalty. The ext3 filesystem does not enable write barriers by default. Be sure to enable barriers unless your disks are battery-backed one way or another. Otherwise, you risk filesystem corruption in case of power failure.
commit=nrsec Sync all data and metadata every nrsec seconds. The default value is 5 seconds. Zero means default.
user_xattr Enable Extended User Attributes, as per the attr command.
acl Enable POSIX Access Control Lists.

Filesystem-Specific mount Options: ext4

The ext4 filesystem is an advanced level of the ext3 filesystem which incorporates scalability and reliability enhancements for supporting large filesystem.

The options journal_dev, noload, data, commit, orlov, oldalloc, [no]user_xattr [no]acl, bsddf, minixdf, debug, errors, data_err, grpid, bsdgroups, nogrpid, sysvgroups, resgid, resuid, sb, quota, noquota, grpquota, and usrquota are backward compatible with ext3 or ext2.

Mount options for ext4:

journal_checksum Enable checksumming of the journal transactions. This allows the recovery code in e2fsck and the kernel to detect corruption in the kernel. It is a compatible change and is ignored by older kernels.
journal_async_commit Commit block can be written to disk without waiting for descriptor blocks. If enabled older kernels cannot mount the device. This enables 'journal_checksum' internally.
journal=update Update the ext4 filesystem's journal to the current format.
This enables/disables the use of write barriers in the jbd code. barrier=0 disables, barrier=1 enables. This also requires an IO stack which can support barriers, and if jbd gets an error on a barrier write, it disables again with a warning. Write barriers enforce proper on-disk ordering of journal commits, making volatile disk write caches safe to use, at some performance penalty. If your disks are battery-backed in one way or another, disabling barriers may safely improve performance. The mount options "barrier" and "nobarrier" can also be used to enable or disable barriers, for consistency with other ext4 mount options.

The ext4 filesystem enables write barriers by default.
inode_readahead=n This tuning parameter controls the maximum number of inode table blocks that ext4's inode table readahead algorithm pre-reads into the buffer cache. The default value is 32 blocks.
stripe=n Number of filesystem blocks that mballoc tries to use for allocation size and alignment. For RAID5/6 systems this should be the number of data disks multipled by the RAID chunk size in filesystem blocks.
delalloc Deferring block allocation until write-out time.
nodelalloc Disable delayed allocation. Blocks are allocated when data is copied from user to page cache.
max_batch_time=usec Maximum amount of time ext4 should wait for additional filesystem operations to be batch together with a synchronous write operation. Since a synchronous write operation is going to force a commit and then a wait for the I/O complete, it doesn't cost much, and is a huge throughput win, we wait for a small amount of time to see if any other transactions can piggyback on the synchronous write. The algorithm used is designed to automatically tune for the speed of the disk, by measuring the amount of time (on average) that it takes to finish committing a transaction. Call this time the "commit time". If the time that the transaction was running is less than the commit time, ext4 tries sleeping for the commit time to see if other operations joins the transaction. The commit time is capped by the max_batch_time, which defaults to 15000 us (15 ms). This optimization can be turned off entirely by setting max_batch_time to 0.
min_batch_time=usec This parameter sets the commit time (as described above) to be at least min_batch_time. It defaults to zero microseconds. Increasing this parameter may improve the throughput of multi-threaded, synchronous workloads on very fast disks, at the cost of increasing latency.
journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the highest priority) which should be used for I/O operations submitted by kjournald2 during a commit operation. This defaults to 3, which is a slightly higher priority than the default I/O priority.
abort Simulate the effects of calling ext4_abort() for debugging purposes. This is normally used while remounting a filesystem that is already mounted.
auto_da_alloc|noauto_da_alloc Many broken applications don't use fsync() when replacing existing files via patterns such as:

fd = open("")write(fd,..)close(fd)rename("", "foo")
or worse yet:

fd = open("foo", O_TRUNC)write(fd,..)close(fd).
If auto_da_alloc is enabled, ext4 detects the replace-via-rename and replace-via-truncate patterns and force that any delayed allocation blocks are allocated such that at the next journal commit, in the default data=ordered mode, the data blocks of the new file are forced to disk before the rename() operation is committed. This provides roughly the same level of guarantees as ext3, and avoids the "zero-length" problem that happens when a system crashes before the delayed allocation blocks are forced to disk.
discard/nodiscard Controls whether ext4 should issue discard/TRIM commands to the underlying block device when blocks are freed. This is useful for SSD devices and sparse/thinly-provisioned LUNs, but it is off by default until sufficient testing was done.
nouid32 Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only store and expect 16-bit values.
resize Allows to resize filesystem to the end of the last existing block group, further resize has to be done with resize2fs either online, or offline. It can be used only with conjunction with remount.
block_validity/noblock_validity These options allow enabling/disabling the in-kernel facility for tracking filesystem metadata blocks within internal data structures. This allows multi-block allocator and other routines to quickly locate extents which might overlap with filesystem metadata blocks. This option is intended for debugging purposes and since it negatively affects the performance, it is off by default.
dioread_lock/dioread_nolock Controls whether or not ext4 should use the DIO read locking. If the dioread_nolock option is specified ext4 allocates uninitialized extent before buffer write and convert the extent to initialized after IO completes. This approach allows ext4 code to avoid using inode mutex (mutual exclusion), which improves scalability on high speed storages. However, this does not work with data journaling and dioread_nolock option is ignored with kernel warning. Note that dioread_nolock code path is only used for extent-based files. Because of the restrictions this options comprises it is off by default (e.g., dioread_lock).
i_version Enable 64-bit inode version support. This option is off by default.

Filesystem-Specific mount Options: fat

File Allocation Table ("fat") is a legacy filesystem. Originally designed for use on floppy disks, it is simple and robust, but lacks the advanced features, performance, reliability and scalability of modern filesystems.

For many years it was the standard filesystem of Microsoft's MS-DOS and Windows 9x line of operating systems.

Mount options for fat:

blocksize={512|1024|2048} Set blocksize (default 512). This option is obsolete.
uid=value, gid=value Set the owner and group of all files. Default: the uid and gid of the current process.
umask=value Set the umask (the bitmask of the permissions that are not present). The default is the umask of the current process. The value is given in octal.
dmask=value Set the umask applied to directories only. The default is the umask of the current process. The value is given in octal.
fmask=value Set the umask applied to regular files only. The default is the umask of the current process. The value is given in octal.

This option controls the permission check of mtime/atime. value can be:

20 If current process is in group of file's group ID, you can change timestamp.
2 Other users can change timestamp.
The default is set from the dmask option. (If the directory is writable, utime is also allowed, i.e., ~dmask & 022)

Normally utime( ) checks current process is owner of the file, or it has CAP_FOWNER capability. But FAT filesystem doesn't have uid/gid on disk, so normal check is too inflexible. With this option you can relax it.

Three different levels of pickiness can be chosen:

r[elaxed] Upper and lowercase are accepted and equivalent, long name parts are truncated (e.g., verylongname.foobar becomes, leading and embedded spaces are accepted in each name part (name and extension).
n[ormal] Like "relaxed", but many special characters (*, ?, <, spaces, etc.) are rejected. This is the default.
s[trict] Like "normal", but names may not contain long parts and special characters that are sometimes used on Linux, but are not accepted by MS-DOS are rejected. (+, =, spaces, etc.)
codepage=value Sets the codepage for converting to shortname characters on FAT and VFAT filesystems. By default, codepage 437 is used.

The fat filesystem can perform CR/LF <--> NL (MS-DOS text format to UNIX text format) conversion in the kernel. The following conversion modes are available:

binary no translation is performed. This is the default.
text CRLF<-->NL translation is performed on all files.
auto CRLF<-->NL translation is performed on all files that don't have a "well-known binary" extension. The list of known extensions is at the beginning of fs/fat/misc.c. As of 2.0, the list is:

exe, com, bin, app, sys, drv, ovl, ovr, obj, lib, dll, pif, arc, zip, lha, lzh, zoo, tar, z, arj, tz, taz, tzp, tpz, gz, tgz, deb, gif, bmp, tif, gl, jpg, pcx, tfm, vf, gf, pk, pxl, dvi.

Programs that do computed lseeks won't like in-kernel text conversion. Several people have had their data ruined by this translation. You have been warned!

For filesystems mounted in binary mode, a conversion tool (fromdos/todos) is available, so ultimately this option is obsolete.
cvf_format=module Forces the driver to use the CVF (Compressed Volume File) module module instead of auto-detection. If the kernel supports kmod, the cvf_format=xxx option also controls on-demand CVF module loading. This option is obsolete.
cvf_option=option Option passed to the CVF module. This option is obsolete.
debug Turn on the debug flag. A version string and a list of filesystem parameters are printed (these data are also printed if the parameters appear to be inconsistent).
fat={12|16|32} Specify a 12, 16 or 32-bit FAT. This overrides the automatic FAT type detection routine, so use it with caution!
iocharset=value Character set to use for converting between 8 bit characters and 16 bit Unicode characters. The default is iso8859-1. Long file names are stored on disk in Unicode format.
tz=UTC This option disables the conversion of timestamps between local time (as used by Windows on FAT) and UTC (which Linux uses internally). This is particularly useful when mounting devices (like digital cameras) that are set to UTC to avoid the pitfalls of local time.
quiet Turn on the quiet flag. Attempts to chown or chmod files do not return errors, although they fail. Use with caution!
showexec If set, the execute permission bits of the file are only allowed if the extension part of the name is .EXE, .COM, or .BAT. Not set by default.
sys_immutable If set, ATTR_SYS attribute on FAT is handled as IMMUTABLE flag on Linux. Not set by default.
flush If set, the filesystem tries to flush to disk earlier than normal. Not set by default.
usefree Use the "free clusters" value stored on FSINFO. It'll be used to determine number of free clusters without scanning disk. But it's not used by default, because recent Windows don't update it correctly in some case. If you are sure the "free clusters" on FSINFO is correct, by this option you can avoid scanning disk.
dots, nodots, dotsOK=[yes|no] Various misguided attempts to force Unix or DOS file naming conventions onto a FAT filesystem. I'm sure they meant well, but you probably won't use this option.

Filesystem-Specific mount Options: hfs

Hierarchical File System ("hfs") is a proprietary file system for MacOS. As of MacOS X version 10.6, Apple no longer supports writing to HFS volumes, although they can still be mounted as read-only.

Mount options for hfs:

creator=cccc, type=cccc Set the creator/type values as shown by the MacOS finder used for creating new files. Default value is '????'.
uid=n, gid=n Set the owner and group of all files. Default: the uid and gid of the current process.
dir_umask=n, file_umask=n, umask=n Set the umask used for all directories, all regular files, or all files and directories. Defaults to the umask of the current process.
session=n Select the CD-ROM session to mount. Defaults to leaving that decision to the CD-ROM driver. This option fails with anything but a CD-ROM as underlying device.
part=n Select partition number n from the device. Only makes sense for CD-ROMs. Defaults to not parsing the partition table at all.
quiet Don't complain about invalid mount options.

Filesystem-Specific mount Options: hpfs

High Performance File System ("hpfs") is a filesystem developed by Microsoft and IBM specifically for the OS/2 operating system. It was designed to improve upon the FAT filesystem.

Mount options for hpfs:

uid=value, gid=value Set the owner and group of all files. Default: the uid and gid of the current process.
umask=value Set the umask (the bitmask of the permissions that are not present). The default is the umask of the current process. The value is given in octal.
case={lower|asis} Convert all files names to lowercase, or leave them as-is. (Default: case=lower.)
conv={binary|text|auto} For conv=text, delete some random CRs (in particular, all followed by NL) when reading a file. For conv=auto, make a best guess between conv=binary and conv=text. For conv=binary, only read what is in the file. This is the default.
nocheck Do not abort mounting when certain consistency checks fail.

Filesystem-Specific mount Options: iso9660

ISO 9660 is a standard describing a filesystem structure to be used on CD-ROMs, and on some DVDs.

Normal iso9660 file names appear in a 8.3 format (i.e., DOS-like restrictions on file name length, where the file name has a maximum of eight characters, a dot, and then a maximum three-character extension), and in addition all characters are in uppercase. Also, there is no field for file ownership, protection, number of links, provision for block/character devices, etc.

"Rock Ridge" is an extension to iso9660 that provides all these Unix-like features. Basically, in iso9660 there are extensions to each directory record that supply all the additional information, and when Rock Ridge is in use, the filesystem is indistinguishable from a normal UNIX filesystem (except that it is read-only).

Mount options for iso9660:

norock Disable the use of Rock Ridge extensions, even if available. Compare with map, below.
nojoliet Disable the use of Microsoft Joliet extensions, even if available. Compare with map, below.
check={r[elaxed]|s[trict]} With check=relaxed, a file name is first converted to lowercase before doing the look up. This is probably only meaningful together with norock and map=normal. Default: check=strict.
uid=value, gid=value Give all files in the filesystem the indicated user or group id, possibly overriding the information found in the Rock Ridge extensions. Default: uid=0,gid=0.
map={n[ormal]|o[ff]|a[corn]} For non-Rock Ridge volumes, normal name translation maps upper to lowercase ASCII, drops a trailing ';1', and converts ';' to '.'. With map=off no name translation is done. See norock, above. Default: map=normal. Note: map=acorn is like map=normal but also applies Acorn extensions if present.
mode=value For non-Rock Ridge volumes, give all files the indicated mode. Default: read permission for everybody. Since Linux 2.1.37, administrators no longer need to specify the mode in decimal; octal is indicated by a leading 0.
unhide Also, show hidden and associated files. If the ordinary files and the associated or hidden files have the same file names, this may make the ordinary files inaccessible.
block={512|1024|2048} Set the block size to the indicated value. Default: block=1024.
conv={a[uto]|b[inary]|m[text]|t[ext]} Default: conv=binary. Since Linux 1.3.54 this option has no effect anymore. And non-binary settings used to be very dangerous, possibly leading to silent data corruption.
cruft If the high byte of the file length contains other garbage, set this mount option to ignore the high order bits of the file length. This implies that a file cannot be larger than 16 MB.
session=x Select number of session on multisession CD. (Since 2.3.4.)
sbsector=xxx Session begins from sector xxx. (Since 2.3.4.)

The following options are the same as for vfat and specifying them only makes sense when using discs encoded using Microsoft's Joliet extensions:

iocharset=value Character set to use for converting 16 bit Unicode characters on CD to 8 bit characters. The default is iso8859-1.
utf8 Convert 16 bit Unicode characters on CD to UTF-8.

Filesystem-Specific mount Options: jfs

Journaled File System ("jfs") is a journaled 64-bit file system created by IBM. There are versions for Linux, AIX, and OS/2, among other operating systems.

Mount options for jfs are as follows:

iocharset=name Character set to use for converting from Unicode to ASCII. The default is to do no conversion. Use iocharset=utf8 for UTF-8 translations. This requires CONFIG_NLS_UTF8 to be set in the kernel .config file.
resize=value Resize the volume to value blocks. JFS only supports growing a volume, not shrinking it. This option is only valid during a remount, when the volume is mounted read-write. The resize keyword with no value grows the volume to the full size of the partition.
nointegrity Do not write to the journal. The primary use of this option is to allow for higher performance when restoring a volume from backup media. The integrity of the volume is not guaranteed if the system abnormally abends.
integrity Default. Commit metadata changes to the journal. Use this option to remount a volume where the nointegrity option was previously specified to restore normal behavior.
errors={continue|remount-ro|panic} Define the behavior when an error is encountered. Either ignore errors and mark the filesystem erroneous and continue, or remount the filesystem read-only, or panic and halt the system.
noquota|quota|usrquota|grpquota These options are accepted but ignored.

Filesystem-Specific mount Options: ntfs

NTFS is a file system developed by Microsoft for its Windows NT operating system.

Mount options for ntfs:

iocharset=name Character set to use when returning file names. Unlike VFAT, NTFS suppresses names that contain nonconvertible characters. This option is deprecated.
nls=name New name for the option earlier called iocharset.
utf8 Use UTF-8 for converting file names.
uni_xlate={0|1|2} For 0 (or 'no' or 'false'), do not use escape sequences for unknown Unicode characters. For 1 (or 'yes' or 'true') or 2, use vfat-style 4-byte escape sequences starting with ":". Here 2 give a little-endian encoding and 1 a byteswapped big-endian encoding.
posix=[0|1] If enabled (posix=1), the filesystem distinguishes between upper and lowercase. The 8.3 alias names are presented as hard links instead of being suppressed. This option is obsolete.
uid=value, gid=value, umask=value Set the file permission on the filesystem. The umask value is given in octal. By default, the files are owned by root and not readable by somebody else.

Filesystem-Specific mount Options: proc

proc (or "procfs") is a special filesystem on Unix-like operating systems which presents running processes in a hierarchical file-like structure. It is often mounted at the location /proc at boot time.

Mount options for proc:

uid=value, gid=value These options are recognized by mount, but have no function.

Filesystem-Specific mount Options: reiserfs

Reiserfs is a general-purpose journaled filesystem. It was the first journaled filesystem supported by the Linux kernel, and was once the default filesystem on SUSE Linux, before ext3 became the default.

Mount options for reiserfs:

conv Instructs version 3.6 reiserfs software to mount a version 3.5 filesystem, using the 3.6 format for newly created objects. This filesystem is longer be compatible with reiserfs 3.5 tools.

Choose which hash function reiserfs use to find files within directories:

rupasov A hash invented by Yury Yu. Rupasov. It is fast and preserves locality, mapping lexicographically close file names to close hash values. This option should not be used, as it causes a high probability of hash collisions.
tea A Davis-Meyer function which uses hash-permuting bits in the name. It gets high randomness and, therefore, low probability of hash collisions at some CPU cost. This may be used if EHASHCOLLISION errors are experienced with the r5 hash.
r5 A modified version of the "rupasov" hash. It is used by default, and is the best choice unless the filesystem has huge directories and unusual file-name patterns.
detect Instructs mount to detect which hash function is in use by examining the filesystem being mounted, and to write this information into the reiserfs superblock. This is only useful on the first mount of an old-format filesystem.
hashed_relocation Tunes the block allocator. This may provide performance improvements in some situations.
no_unhashed_relocation Tunes the block allocator. This may provide performance improvements in some situations.
noborder Disable the border allocator algorithm. This may provide performance improvements in some situations.
nolog Disable journalling. This provides slight performance improvements in some situations at the cost of losing reiserfs's fast recovery from crashes. Even with this option turned on, reiserfs still performs all journalling operations, save for actual writes into its journalling area. Implementation of nolog is a work in progress.
notail By default, reiserfs stores small files and 'file tails' directly into its tree. This confuses some utilities such as LILO. This option is used to disable packing of files into the tree.
replayonly Replay the transactions that are in the journal, but do not actually mount the filesystem. Mainly used by reiserfsck, which checks the integrity of the filesystem.
resize=number A remount option which permits online expansion of reiserfs partitions. Instructs reiserfs to assume that the device has number blocks. This option is designed for use with devices that are under LVM (logical volume management).
user_xattr Enable Extended User Attributes.
acl Enable POSIX Access Control Lists.
barrier=none, barrier=flush This enables/disables the use of write barriers in the journaling code. barrier=none disables it, barrier=flush enables it. Write barriers enforce proper on-disk ordering of journal commits, making volatile disk write caches safe to use, at some performance penalty. The reiserfs filesystem does not enable write barriers by default. Be sure to enable barriers unless your disks are battery-backed one way or another. Otherwise, you risk filesystem corruption in case of power failure.

Filesystem-Specific mount Options: tmpfs

tmpfs is the name of a temporary file system used on many Unix-like filesystems. It is mounted and structured like a disk-based filesystem, but resides in volatile memory space, similar to a RAM disk. Mount options for tmpfs:

size=nbytes Override default maximum size of the filesystem. The size is given in bytes, and rounded up to entire pages. The default is half of the memory. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your physical RAM: the default, when neither size nor nr_blocks is specified, is size=50%.
nr_blocks=... The same as size, but in blocks of PAGE_CACHE_SIZE.
nr_inodes=... The maximum number of inodes for this instance. The default is half of the number of your physical RAM pages, or (on a machine with "highmem") the number of lowmem RAM pages, whichever is the lower.

The tmpfs mount options for sizing (size, nr_blocks, and nr_inodes) accept a suffix k, m or g for Ki, Mi, Gi (binary kilo, mega and giga) and can be changed on remount.

mode= Set initial permissions of the root directory.
uid= The user id.
gid= The group id.

Set the NUMA (non-uniform memory access) memory allocation policy for all files in that instance (if the kernel option CONFIG_NUMA is enabled). It can be adjusted on the fly via 'mount -o remount ...'.

default prefers to allocate memory from the local node.
prefer:Node prefers to allocate memory from the given node Node
bind:NodeList allocates memory only from nodes in NodeList
interleave prefers to allocate from each node in turn
interleave:NodeList allocates from each node of NodeList in turn.
The NodeList format is a comma-separated list of decimal numbers and ranges, a range being two hyphen-separated decimal numbers, the smallest and largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15

Note that trying to mount a tmpfs with an mpol option fails if the running kernel does not support NUMA; and fails if its nodelist specifies a node that is not online. If your system relies on that tmpfs being mounted, but from time to time runs a kernel built without NUMA capability (perhaps a safe recovery kernel), or with fewer nodes online, then it is advisable to omit the mpol option from automatic mount options. It can be added later, when the tmpfs is already mounted on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.

Filesystem-Specific mount Options: ubifs

UBIFS is a flash file system which works on top of UBI volumes. Note that atime is not supported and is always turned off.

The device name may be specified as:

ubiX_Y UBI device number X, volume number Y
ubiY UBI device number 0, volume number Y
ubiX:NAME UBI device number X, volume with name NAME
ubi:NAME UBI device number 0, volume with name NAME

An alternative "!" separator may be used instead of ":".

Mount options for ubifs:

bulk_read Enable bulk-read. VFS read-ahead is disabled because it slows down the file system. Bulk-Read is an internal optimization. Some flashes may read faster if the data are read at one go, rather than at several read requests. For example, OneNAND can do "read-while-load" if it reads more than one NAND page.
no_bulk_read Do not bulk-read. This is the default.
chk_data_crc Check data CRC-32 checksums. This is the default.
no_chk_data_crc Do not check data CRC-32 checksums. With this option, the filesystem does not check CRC-32 checksum for data, but it does check it for the internal indexing information. This option only affects reading, not writing. CRC-32 is always calculated when writing the data.
compr={none|lzo|zlib} Select the default compressor that is used when new files are written. It is still possible to read compressed files if mounted with the none option.

Filesystem-Specific mount Options: udf

udf is the "Universal Disk Format" filesystem defined by the Optical Storage Technology Association, and is often used for DVD-ROM. See also iso9660, above.

Mount options for udf:

gid= Set the default group.
umask= Set the default umask. The value is given in octal.
uid= Set the default user.
unhide Show otherwise hidden files.
undelete Show deleted files in lists.
nostrict Unset strict conformance.
iocharset Set the NLS character set.
bs= Set the block size. May not work unless 2048.
novrs Skip volume sequence recognition.
session= Set the CD-ROM session counting from 0. Default: last session.
anchor= Override standard anchor location. Default: 256.
volume= Override the VolumeDesc location. (unused)
partition= Override the PartitionDesc location. (unused)
lastblock= Set the last block of the filesystem.
fileset= Override the fileset block location. (unused)
rootdir= Override the root directory location. (unused)

Filesystem-Specific mount Options: ufs

ufs is the Unix File System. It is used in many Unix and Unix-like filesystems, and is a descendent of the filesystem used in Version 7 Unix.


UFS is a filesystem widely used in different operating systems; there are significant differences among implementations. Features of some implementations are undocumented, so its hard to recognize the type of ufs automatically. That's why the user must specify the type of ufs by mount option. Possible values are:

old Old format of ufs; this is the default, and is read-only, so don't forget to give the -r option as well.
44bsd For filesystems created by a BSD-like system (NetBSD, FreeBSD, OpenBSD).
ufs2 Used in FreeBSD 5.x; supported as read-write.
5xbsd Synonym for ufs2.
sun For filesystems created by SunOS, or Solaris on Sparc.
sunx86 For filesystems created by Solaris on x86.
hp For filesystems created by HP-UX; read-only.
nextstep For filesystems created by NeXTSTEP (on NeXT station); currently read-only.
nextstep-cd For NextStep CD-ROMs (block_size == 2048); read-only.
openstep For filesystems created by OpenStep (currently read-only). The same filesystem type is also used by macOS X.

Set behavior on encountering an error:

panic If an error is encountered, cause a kernel panic.
[lock|umount|repair] These mount options don't do anything at present; when an error is encountered only a console message is printed.

Filesystem-Specific mount Options: usbfs

The usbfs filesystem is dynamically generated, similar to proc (see The Loop Device, below). It is used primarily for USB devices on Linux and other Unix-like filesystems.

Mount options for usbfs:

devuid=uid, devgid=gid, devmode=mode Set the owner and group and mode of the device files in the usbfs filesystem (default: uid=gid=0, mode=0644). The mode is given in octal.
busuid=uid, busgid=gid, busmode=mode Set the owner and group and mode of the bus directories in the usbfs filesystem (default: uid=gid=0, mode=0555). The mode is given in octal.
listuid=uid, listgid=gid, listmode=mode Set the owner and group and mode of the file devices (default: uid=gid=0, mode=0444). The mode is given in octal.

Filesystem-Specific mount Options: vfat

Virtual FAT, or vfat, is closely related to the FAT file system. Options that are valid for fat (see above) are also valid for vfat, except for the "dot" options. Additional options are as follows. Mount options for vfat:

uni_xlate Translate unhandled Unicode characters to special escaped sequences. This lets you back up and restore file names that are created with any Unicode characters. Without this option, a '?' is used when no translation is possible. The escape character is ':' because it is otherwise illegal on the vfat filesystem. The escape sequence that gets used, where u is the unicode character, is: ':', (u & 0x3f), ((u>>6) & 0x3f), (u>>12).
posix Allow two files with names that only differ in case. This option is obsolete.
nonumtail First try to make a short name without sequence number, before trying name~num.ext.
utf8 UTF8 is the filesystem-safe 8-bit encoding of Unicode that is used by the console. It can be enabled for the filesystem with this option or disabled with utf8=0, utf8=no or utf8=false. If 'uni_xlate' gets set, UTF8 gets disabled.

Defines the behavior for creation and display of file names which fit into 8.3 characters. If a long name for a file exists, it is always the preferred display. There are four modes:

lower Force the short name to lowercase upon display; store a long name when the short name is not all uppercase.
win95 Force the short name to uppercase upon display; store a long name when the short name is not all uppercase.
winnt Display the shortname as is; store a long name when the short name is not all lowercase or all uppercase.
mixed Display the short name as is; store a long name when the short name is not all uppercase. This mode is the default since Linux 2.6.32.

Filesystem-Specific mount Options: xfs

xfs is a 64-bit, journaled filesystem developed by Silicon Graphics, Inc. It was the default filesystem in IRIX for many years, and has also been ported to the Linux kernel. xfs excels at parallel I/O, data consistency, and overall filesystem performance. It is well suited for real-time applications, due to a unique feature that allows it to maintain guaranteed data I/O bandwidth.

Mount options for xfs:

allocsize=size Sets the buffered I/O end-of-file preallocation size when doing delayed allocation writeout (default size is 64 KiB). Valid values for this option are page size (often 4 KiB) through to 1 GiB, inclusive, in power-of-2 increments.
<attr2> | noattr2 The options enable/disable (default is enabled) an "opportunistic" improvement to be made in the way inline extended attributes are stored on-disk. When the new form is used for the first time (by setting or removing extended attributes) the on-disk superblock feature bit field is updated to reflect this format being in use.
barrier Enables the use of block layer write barriers for writes into the journal and unwritten extent conversion. This allows for drive level write caching to be enabled, for devices that support write barriers.
dmapi Enable the DMAPI (Data Management API) event callouts. Use with the mtpt option.
grpid|bsdgroups, nogrpid|sysvgroups These options define what group ID a newly created file gets. When grpid is set, it takes the group ID of the directory where it is created; otherwise (the default) it takes the fsgid of the current process, unless the directory has the setgid bit set, where case it takes the gid from the parent directory, and also gets the setgid bit set if it's a directory itself.
ihashsize=value Sets the number of hash buckets available for hashing the in-memory inodes of the specified mount point. If a value of zero is used, the value selected by the default algorithm is displayed in /proc/mounts.
ikeep|noikeep When inode clusters are emptied of inodes, keep them around on the disk (ikeep) - this is the traditional XFS behavior and is still the default for now. Using the noikeep option, inode clusters are returned to the free space pool.
inode64 Indicates that XFS is allowed to create inodes at any location in the filesystem, including those which result in inode numbers occupying more than 32 bits of significance. This is provided for backward compatibility, but causes problems for backup applications that cannot handle large inode numbers.
largeio|nolargeio If nolargeio is specified, the optimal I/O reported in st_blksize by stat is as small as possible to allow user applications to avoid inefficient read/modify/write I/O. If largeio is specified, a filesystem with a swidth specified returns the swidth value (in bytes) in st_blksize. If the filesystem does not have a swidth specified but does specify an allocsize then allocsize (in bytes) is returned instead. If neither of these two options are specified, then filesystem behaves as if nolargeio was specified.
logbufs=value Set the number of in-memory log buffers. Valid numbers range from 2-8 inclusive. The default value is 8 buffers for any recent kernel.
logbsize=value Set the size of each in-memory log buffer. Size may be specified in bytes, or in kilobytes with a "k" suffix. Valid sizes for version 1 and version 2 logs are 16384 (16k) and 32768 (32k). Valid sizes for version 2 logs also include 65536 (64k), 131072 (128k) and 262144 (256k). The default value for any recent kernel is 32768.
logdev=device, rtdev=device Use an external log (metadata journal) and/or real-time device. An XFS filesystem has up to three parts: a data section, a log section, and a real-time section. The real-time section is optional, and the log section can be separate from the data section or contained within it.
mtpt=mountpoint Use with the dmapi option. The value specified here is included in the DMAPI mount event, and should be the path of the actual mountpoint that is used.
noalign Data allocations aren't aligned at stripe unit boundaries.
noatime Access timestamps are not updated when a file is read.
norecoverywil The filesystem is mounted without running log recovery. If the filesystem was not cleanly unmounted, it is likely to be inconsistent when mounted in norecovery mode. Some files or directories may not be accessible because of this. Filesystems mounted norecovery must be mounted read-only or the mount fails.
nouuid Don't check for double mounted filesystems using the filesystem uuid. This is useful to mount LVM snapshot volumes.
osyncisosync Make O_SYNC writes implement true O_SYNC. WITHOUT this option, Linux XFS behaves as if an osyncisdsync option is used, which makes writing to files opened with the O_SYNC flag set behave as if the O_DSYNC flag had been used instead. This can result in better performance without compromising data safety. However, if this option is not in effect, timestamp updates from O_SYNC writes can be lost if the system crashes. If timestamp updates are critical, use the osyncisosync option.
uquota|usrquota|uqnoenforce|quota User disk quota accounting enabled, and limits (optionally) enforced.
gquota|grpquota|gqnoenforce Group disk quota accounting enabled and limits (optionally) enforced.
pquota|prjquota|pqnoenforce Project disk quota accounting enabled and limits (optionally) enforced.
sunit=value, swidth=value Used to specify the stripe unit and width for a RAID device or a stripe volume. The value must be specified in 512-byte block units. If this option is not specified and the filesystem was made on a stripe volume or the stripe width or unit were specified for the RAID device at mkfs time, then the mount system call restores the value from the superblock. For filesystems that are made directly on RAID devices, these options can override the information in the superblock if the underlying disk layout changes after the filesystem was created. The swidth option is required if the sunit option is specified, and must be a multiple of the sunit value.
swalloc Data allocations are rounded up to stripe width boundaries when the current end of file is being extended and the file size is larger than the stripe width size.

The Loop Device

One further possible type is a mount via the loop device. A loop device is a "pseudo device" which allows a file to be mounted as a filesystem. For example, the command:

mount /tmp/disk.img /mnt -t vfat -o loop=/dev/loop

sets up the loop device /dev/loop3 to correspond to the file /tmp/disk.img, and then mount this device on /mnt.

If no explicit loop device is mentioned (but only an option '-o loop' is given), then mount tries to find some unused loop device and use that; for example:

mount /tmp/disk.img /mnt -o loop

The mount command automatically creates a loop device from a regular file if a filesystem type is not specified or the filesystem is known for libblkid, for example:

mount /tmp/disk.img /mnt
mount -t ext3 /tmp/disk.img /mnt

This type of mount knows about four options, namely loop, offset, sizelimit and encryption, that are really options to losetup. If the mount requires a passphrase, you are prompted for one unless you specify a file descriptor to read from instead with the --pass-fd option. These options can be used in addition to those specific to the filesystem type.

Since Linux 2.6.25 is supported auto-destruction of loop devices and then any loop device allocated by mount is freed by umount independently on /etc/mtab. You can also free a loop device by hand, using 'losetup -d' or 'umount -d'.


The following options can be provided when umounting a device with umount:

-V Print version information and exit.
-h Print a help message and exit.
-v Run in verbose mode.
-n Unmount without writing in /etc/mtab.
-r In case unmounting fails, try to remount read-only.
-d In case the unmounted device was a loop device, also free this loop device.
-i Don't call the /sbin/umount.filesystem helper even if it exists. By default, /sbin/umount.filesystem helper is called if one exists.
-a All the file systems described in /etc/mtab are to be unmounted. With umount version 2.7 and later, the proc filesystem is not unmounted.
-t vfstype Indicate that the actions should only be taken on file systems of the specified type. More than one type may be specified in a comma separated list. The list of file system types can be prefixed with no to specify the file system types on which no action should be taken.
-O options Indicate that the actions should only be taken on file systems with the specified options in /etc/fstab. More than one option type may be specified in a comma separated list. Each option can be prefixed with no to specify options for which no action should be taken.
-f Force unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.)
-l "Lazy" unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)
--no-canonicalize Don't canonicalize (standardize the textual representation of) paths.
--fake Causes everything to be done except for the actual system call; this "fakes" unmounting the filesystem. It can remove entries from /etc/mtab that were unmounted earlier with the -n option.

Return Codes

mount returns one of the following values to indicate what happened during its execution:

0 success.
1 incorrect invocation or permissions.
2 a system error occurred (out of memory, cannot fork, no more loop devices).
4 internal mount bug was encountered.
8 interrupted by user.
16 problems writing or locking /etc/mtab.
32 mount failure.
64 At least one mount succeeded, but not all.


mount makes use of the following files:

/etc/fstab filesystem table
/etc/mtab table of filesystems that are mounted. This file is also used by umount.
/etc/mtab~ lock file
/etc/mtab.tmp temporary file
/etc/filesystems list of filesystem types to try


mount -t type devicename destination_directory

Mount the device of devicename devicename, of type type, at filesystem location destination_directory.

mount -t iso9660 -o ro /dev/cdrom /mnt

Mount a CD-ROM in the directory /mnt. iso9660 is the standard file system for CD-ROMs, -o ro tells mount to mount it as a read-only filesystem. /mnt must already exist for this command to be successful.


Display all current mounts. Output appears similar to the following:

/dev/loop0 on / type ext4 (rw,errors=remount-ro)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
none on /sys/fs/cgroup type tmpfs (rw)
none on /sys/fs/fuse/connections type fusectl (rw)
none on /sys/kernel/debug type debugfs (rw)
none on /sys/kernel/security type securityfs (rw)
udev on /dev type devtmpfs (rw,mode=0755)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)
/dev/sda3 on /host type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other,blksize=4096)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
gvfsd-fuse on /run/user/lightdm/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,user=lightdm)
mount -l -t tmpfs

List all current mounts of type tmpfs. Output resembles the following:

none on /sys/fs/cgroup type tmpfs (rw)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)
mount -a

Mount all filesystems listed in the filesystem table file /etc/fstab.

mount -o loop disk1.iso /mnt/disk

Mount the ISO Image file disk1.iso as a loop device (see above) at the directory /mnt/disk. The directory /mnt/disk must already exist for this command to be successful.

eject — Eject removable media.
losetup — Set up and control loop devices.