mount Man page

MOUNT(8) System Administration MOUNT(8)


mount – mount a filesystem


mount [-l|-h|-V]

mount -a [-fFnrsvw] [-t fstype] [-O optlist]

mount [-fnrsvw] [-o options] device|dir

mount [-fnrsvw] [-t fstype] [-o options] device dir


All files accessible in a Unix system are arranged in one big tree, the
file hierarchy, rooted at /. These files can be spread out over sev‐
eral devices. The mount command serves to attach the filesystem found
on some device to the big file tree. Conversely, the umount(8) command
will detach 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)
and 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 the directory or the device is given, for example:

mount /dir

then mount looks for a mountpoint (and if not found then for a device)
in the /etc/fstab file. It’s possible to use the –target or –source
options to avoid ambivalent interpretation of the given argument. For

mount –target /mountpoint

The listing.
The listing mode is maintained for backward compatibility only.

For more robust and customizable output use findmnt(8), espe‐
cially in your scripts. Note that control characters in the
mountpoint name are replaced with ‘?’.

The following command lists all mounted filesystems (of type

mount [-l] [-t type]

The option -l adds labels to this listing. See below.

The device indication.
Most devices are indicated by a filename (of a block special
device), like /dev/sda1, but there are other possibilities. For
example, in the case of an NFS mount, device may look like It is also possible to indicate a block spe‐
cial device using its filesystem label or UUID (see the -L and
-U options below), or its partition label or UUID. (Partition
identifiers are supported for example for GUID Partition Tables

Don’t forget that there is no guarantee that UUIDs and labels
are really unique, especially if you move, share or copy the
device. Use lsblk -o +UUID,PARTUUID to verify that the UUIDs
are really unique in your system.

The recommended setup is to use tags (e.g. LABEL=label) rather
than /dev/disk/by-{label,uuid,partuuid,partlabel} udev symlinks
in the /etc/fstab file. Tags are more readable, robust and por‐
table. The mount(8) command internally uses udev symlinks, so
the use of symlinks in /etc/fstab has no advantage over tags.
For more details see libblkid.

Note that mount(8) uses UUIDs as strings. The UUIDs from the
command line or from fstab(5) are not converted to internal
binary representation. The string representation of the UUID
should be based on lower case 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.)

The /etc/fstab, /etc/mtab and /proc/mounts files.
The file /etc/fstab (see fstab(5)), may contain lines describing
what devices are usually mounted where, using which options.
The default location of the fstab(5) file can be overridden with
the –fstab path command-line option (see below for more

The command

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

(usually given in a bootscript) 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. Adding the -F option
will make mount fork, so that the filesystems are mounted simul‐

When mounting a filesystem mentioned in fstab or mtab, it suf‐
fices to specify on the command line only the device, or only
the mount point.

The programs mount and umount traditionally maintained a list of
currently mounted filesystems in the file /etc/mtab. This real
mtab file is still supported, but on current Linux systems it is
better to make it a symlink to /proc/mounts instead, because a
regular mtab file maintained in userspace cannot reliably work
with namespaces, containers and other advanced Linux features.

If no arguments are given to mount, the list of mounted filesys‐
tems is printed.

If you want to override mount options from /etc/fstab you have
to use the -o option:

mount device|dir -o options

and then the mount options from the command line will be
appended to the list of options from /etc/fstab. The usual
behavior is that the last option wins if there are conflicting

The mount program does not read the /etc/fstab file if both
device (or LABEL, UUID, PARTUUID or PARTLABEL) and dir are spec‐
ified. For example, to mount device foo at /dir:

mount /dev/foo /dir

The 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 filesystem.

Thus, given a line

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

any user can mount the iso9660 filesystem found on an inserted
CDROM using the command

mount /dev/cdrom


mount /cd

For more details, see fstab(5). Only the user that mounted a
filesystem can unmount it again. If any user should be able to
unmount it, then 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 e.g. 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 group
of the special file.

The bind mounts.
Since Linux 2.4.0 it is possible to remount part of the file
hierarchy somewhere else. The call is:

mount –bind olddir newdir

or by using this fstab entry:

/olddir /newdir none bind

After this call the same contents are 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 by using:

mount –rbind olddir newdir

Note that the filesystem mount options will remain the same as
those on the original mount point.

mount(8) since v2.27 allow to change the options by passing the
-o option along with –bind for example:

mount –bind,ro foo foo

This feature is not supported by Linux kernel and it is imple‐
mented in userspace by additional remount mount syscall. This
solution is not atomic.

The alternative (classic) way to create a read-only bind mount
is to use remount operation, for example:

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

Note that read-only bind will create a read-only mountpoint (VFS
entry), but the original filesystem superblock will still be
writable, meaning that the olddir will be writable, but the
newdir will be read-only.

It’s impossible to change mount options recursively (for example
b -o rbind,ro).

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

This will cause the contents which previously appeared under
olddir to now be accessible under newdir. The physical location
of the files is not changed. Note that olddir has to be a

Note also that moving a mount residing under a shared mount is
invalid and unsupported. Use findmnt -o TARGET,PROPAGATION to
see the current propagation flags.

The shared subtree operations.
Since Linux 2.6.15 it is possible to mark a mount and its sub‐
mounts as shared, private, slave or unbindable. A shared mount
provides the ability to create mirrors of that mount such that
mounts and unmounts within any of the mirrors propagate to the
other mirror. A slave mount receives propagation from its mas‐
ter, but not vice versa. A private mount carries no propagation
abilities. An unbindable mount is a private mount which cannot
be cloned through a bind operation. The detailed semantics are
documented in Documentation/filesystems/sharedsubtree.txt file
in the kernel source tree.

Supported operations are:

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

The following commands allow 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(8) does not read fstab(5) when a –make-* operation is
requested. All necessary information has to be specified on the
command line.

Note that the Linux kernel does not allow to change multiple
propagation flags with a single mount syscall, and the flags
cannot be mixed with other mount options.

Since util-linux 2.23 the mount command allows to use several
propagation flags together and also together with other mount
operations. This feature is EXPERIMENTAL. The propagation
flags are applied by additional mount syscalls when the pre‐
ceding mount operations were successful. Note that this use
case is not atomic. It is possible to specify the propagation
flags in fstab(5) as mount options (private, slave, shared,
unbindable, rprivate, rslave, rshared, runbindable).

For example:

mount –make-private –make-unbindable /dev/sda1 /foo

is the same as:

mount /dev/sda1 /foo
mount –make-private /foo
mount –make-unbindable /foo



The full set of mount options used by an invocation of mount is deter‐
mined 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.

The command mount does not pass all command-line options to the
/sbin/mount.suffix mount helpers. The interface between mount and the
mount helpers is described below in the section EXTERNAL HELPERS.

Command-line options available for the mount command are:

-a, –all
Mount all filesystems (of the given types) mentioned in fstab
(except for those whose line contains the noauto keyword). The
filesystems are mounted following their order in fstab.

-B, –bind
Remount a subtree somewhere else (so that its contents are
available in both places). See above.

-c, –no-canonicalize
Don’t canonicalize paths. The mount command canonicalizes all
paths (from command line or fstab) by default. This option can
be used together with the -f flag for already canonicalized
absolute paths. The option is designed for mount helpers which
call mount -i. It is strongly recommended to not use this com‐
mand-line option for normal mount operations.

Note that mount(8) does not pass this option to the
/sbin/mount.type helpers.

-F, –fork
(Used in conjunction with -a.) Fork off a new incarnation of
mount for each device. This will do the mounts on different
devices or different NFS servers in parallel. This has the
advantage that it is faster; also NFS timeouts go 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.

-f, –fake
Causes everything to be done except for the actual system call;
if it’s not obvious, this “fakes” mounting the filesystem.
This option is useful in conjunction with the -v flag to deter‐
mine 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 an existing record in
/etc/mtab and fails when the record already exists (with a regu‐
lar non-fake mount, this check is done by the kernel).

-i, –internal-only
Don’t call the /sbin/mount.filesystem helper even if it exists.

-L, –label label
Mount the partition that has the specified label.

-l, –show-labels
Add the labels in 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(8) utility, or for XFS using xfs_admin(8), or for reis‐
erfs using reiserfstune(8).

-M, –move
Move a subtree to some other place. See above.

-n, –no-mtab
Mount without writing in /etc/mtab. This is necessary for exam‐
ple when /etc is on a read-only filesystem.

-O, –test-opts opts
Limit the set of filesystems to which the -a option applies. In
this regard it is like the -t option except that -O is useless
without -a. For example, the command:

mount -a -O no_netdev

mounts all filesystems except those which have the option _net‐
dev 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

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

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

-o, –options opts
Use the specified mount options. The opts argument is a comma-
separated list. For example:

mount LABEL=mydisk -o noatime,nodev,nosuid

For more details, see the FILESYSTEM-INDEPENDENT MOUNT



-R, –rbind
Remount a subtree and all possible submounts somewhere else (so
that its contents are available in both places). See above.

-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 exam‐
ple, ext3 and ext4 will replay the journal if the filesystem is
dirty. To prevent this kind of write access, you may want to
mount an ext3 or ext4 filesystem with the ro,noload mount
options or set the block device itself to read-only mode, see
the blockdev(8) command.

-s Tolerate sloppy mount options rather than failing. This will
ignore mount options not supported by a filesystem type. Not
all filesystems support this option. Currently it’s supported
by the mount.nfs mount helper only.

–source device
If only one argument for the mount command is given then the
argument might be interpreted as target (mountpoint) or source
(device). This option allows to explicitly define that the
argument is the mount source.

–target directory
If only one argument for the mount command is given then the
argument might be interpreted as target (mountpoint) or source
(device). This option allows to explicitly define that the
argument is the mount target.

-T, –fstab path
Specifies an alternative fstab file. If path is a directory
then the files in the directory are sorted by strverscmp;
files that start with “.” or without an .fstab extension are
ignored. The option can be specified more than once. This
option is mostly designed for initramfs or chroot scripts where
additional configuration is specified beyond standard system

Note that mount(8) does not pass the option –fstab to the
/sbin/mount.type helpers, meaning that the alternative fstab
files will be invisible for the helpers. This is no problem for
normal mounts, but user (non-root) mounts always require fstab
to verify the user’s rights.

-t, –types fstype
The argument following the -t is used to indicate the filesystem
type. The filesystem types which are currently supported depend
on the running kernel. See /proc/filesystems and /lib/mod‐
ules/$(uname -r)/kernel/fs for a complete list of the filesys‐
tems. The most common are ext2, ext3, ext4, xfs, btrfs, vfat,
sysfs, proc, nfs and cifs.

The programs mount and umount support filesystem subtypes. The
subtype is defined by a ‘.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).

If no -t option is given, or if the auto type is specified,
mount will try 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 will try to read the file
/etc/filesystems, or, if that does not exist, /proc/filesystems.
All of the filesystem types listed there will be tried, except
for those that are labeled “nodev” (e.g., devpts, proc and nfs).
If /etc/filesystems ends in a line with a single *, mount will
read /proc/filesystems afterwards. While trying, all filesystem
types will be mounted with the mount option silent.

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,
for option -t as well as in an /etc/fstab entry. The list of
filesystem types for option -t can be prefixed with no to spec‐
ify the filesystem types on which no action should be taken.
The prefix no has no effect when specified in an /etc/fstab

The prefix no can be meaningful with the -a option. For exam‐
ple, the command

mount -a -t nomsdos,smbfs

mounts all filesystems except those of type msdos and smbfs.

For most types all the mount program has to do is issue a simple
mount system call, and no detailed knowledge of the filesys‐
tem type is required. For a few types however (like nfs, nfs4,
cifs, smbfs, ncpfs) an ad hoc code is necessary. The nfs, nfs4,
cifs, smbfs, and ncpfs filesystems have a separate mount pro‐
gram. In order to make it possible to treat all types in a uni‐
form way, mount will execute the program /sbin/mount.type (if
that exists) when called with type type. Since different ver‐
sions of the smbmount program have different calling conven‐
tions, /sbin/mount.smbfs may have to be a shell script that sets
up the desired call.

-U, –uuid uuid
Mount the partition that has the specified uuid.

-v, –verbose
Verbose mode.

-w, –rw, –read-write
Mount the filesystem read/write. This is the default. A syn‐
onym is -o rw.

-V, –version
Display version information and exit.

-h, –help
Display help text and exit.



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. Note that filesystems also have per-filesystem specific
default mount options (see for example tune2fs -l output for extN

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 an 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 the noatime feature, so the inode access time is con‐
trolled by kernel defaults. See also the descriptions of the
strictatime and relatime mount options.

Do not update inode access times on this filesystem (e.g., for
faster access on the news spool to speed up news servers). This
works for all inode types (directories too), so implies nodira‐

auto Can be mounted with the -a option.

noauto Can only be mounted explicitly (i.e., the -a option will not
cause the filesystem to be mounted).

context=context, fscontext=/context, defcontext=/context and rootcon‐
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. kernel ver‐
sions. 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=”sys‐

Two other options are fscontext= and defcontext=, both of which
are mutually exclusive of the context option. This means you
can use fscontext and defcontext with each other, 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 filesys‐
tem label is separate from the individual labels on the files.
It represents the entire filesystem for certain kinds of permis‐
sion 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 con‐
text 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 a FS being mounted before that FS or inode becomes vis‐
ible to userspace. This was found to be useful for things like
stateless linux.

Note that the kernel rejects any remount request that includes
the context option, even when unchanged from the current con‐

Warning: the context value might contain commas, in which case
the value has to be properly quoted, otherwise mount(8) will
interpret the comma as a separator between mount options. Don’t
forget that the shell strips off quotes and thus double quoting
is required. For example:

mount -t tmpfs none /mnt -o \

For more details, see selinux(8).

Use the default options: rw, suid, dev, exec, auto, nouser, and

Note that the real set of all default mount options depends on
kernel and filesystem type. See the beginning of this section
for more details.

dev Interpret character or block special devices on the filesystem.

nodev Do not interpret character or block special devices on the file

Update directory inode access times on this filesystem. This is
the default. Directory inode will not be updated when noatime
is set, regardless of this option.

Do not update directory inode access times on this filesystem.
If noatime option is set, this option is not needed.

All directory updates within the filesystem should be done syn‐
chronously. This affects the following system calls: creat,
link, unlink, symlink, mkdir, rmdir, mknod and rename.

exec Permit execution of binaries.

noexec Do not permit direct execution of any binaries on the mounted
filesystem. (Until recently it was possible to run binaries
anyway using a command like /lib/ld*.so /mnt/binary. This trick
fails since Linux 2.4.25 / 2.6.0.)

group Allow an ordinary user to mount the filesystem if one of that
user’s groups matches the group of the device. This option
implies the options nosuid and nodev (unless overridden by sub‐
sequent options, as in the option line group,dev,suid).

Every time the inode is modified, the i_version field will be

Do not increment the i_version inode field.

mand Allow mandatory locks on this filesystem. See fcntl(2).

nomand Do not allow mandatory locks on this filesystem.

The filesystem resides on a device that requires network access
(used to prevent the system from attempting to mount these
filesystems until the network has been enabled on the system).

nofail Do not report errors for this device if it does not exist.

Update inode access times relative to modify or change time.
Access time is only updated if the previous access time was ear‐
lier than the current modify or change time. (Similar to noat‐
ime, but it doesn’t break mutt or other applications that need
to know if a file has been read since the last time it was modi‐

Since Linux 2.6.30, the kernel defaults to the behavior provided
by this option (unless noatime was specified), and the stricta‐
time option is required to obtain traditional semantics. In
addition, since Linux 2.6.30, the file’s last access time is
always updated if it is more than 1 day old.

Do not use the relatime feature. See also the strictatime mount

Allows to explicitly request full atime updates. This makes it
possible for the kernel to default to relatime or noatime but
still allow userspace to override it. For more details about
the default system mount options see /proc/mounts.

Use the kernel’s default behavior for inode access time updates.

Only update times (atime, mtime, ctime) on the in-memory version
of the file inode.

This mount option significantly reduces writes to the inode ta‐
ble for workloads that perform frequent random writes to preal‐
located files.

The on-disk timestamps are updated only when:

– the inode needs to be updated for some change unrelated to
file timestamps

– the application employs fsync, syncfs(2), or sync

– an undeleted inode is evicted from memory

– more than 24 hours have passed since the i-node was written to

Do not use the lazytime feature.

suid Allow set-user-identifier or set-group-identifier bits to take

nosuid Do not allow set-user-identifier or set-group-identifier bits to
take effect.

silent Turn on the silent flag.

loud Turn off the silent flag.

owner Allow an ordinary user to mount the filesystem if that user 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).

Attempt to remount an already-mounted filesystem. This is com‐
monly used to change the mount flags for a filesystem, espe‐
cially to make a readonly filesystem writable. It does not
change device or mount point.

The remount functionality follows the standard way the mount
command works with options from fstab. This means that the
mount command only doesn’t read fstab (or mtab) when both the
device and dir are specified.

mount -o remount,rw /dev/foo /dir

After this call all old mount options are replaced and arbitrary
stuff from fstab (or mtab) is ignored, except the loop= option
which is internally generated and maintained by the mount com‐

mount -o remount,rw /dir

After this call mount reads fstab and merges these options with
the options from the command line (-o). If no mountpoint found
in fstab than remount with unspecified source is allowed.

ro Mount the filesystem read-only.

rw Mount the filesystem read-write.

sync All I/O to the filesystem should be done synchronously. In the
case of media with a 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 the mtab file (or to the private
libmount file in /run/mount on systems without a regular mtab)
so that this same user can unmount the filesystem again. This
option implies the options noexec, nosuid, and nodev (unless
overridden by subsequent options, as in the option line

nouser Forbid an ordinary user to mount the filesystem. This is the
default; it does not imply any other options.

users Allow any user to mount and to unmount the filesystem, even when
some other ordinary user mounted it. This option implies the
options noexec, nosuid, and nodev (unless overridden by subse‐
quent options, as in the option line users,exec,dev,suid).

x-* All options prefixed with “x-” are interpreted as comments or as
userspace application-specific options. These options are not
stored in the mtab file, nor sent to the mount.type helpers nor
to the mount system call. The suggested format is x-app‐
name.option (e.g. x-systemd.automount).

x-mount.mkdir[=mode] Allow to make a target directory (mountpoint). The optional
argument mode specifies the filesystem access mode used for
mkdir in octal notation. The default mode is 0755. This
functionality is supported only for root users.



The following options apply only to certain filesystems. We sort them
by filesystem. They all follow the -o flag.

What options are supported depends a bit on the running kernel. More
info may be found in the kernel source subdirectory Documenta‐

Mount options for adfs
uid=value and gid=value
Set the owner and group of the files in the filesystem (default:

ownmask=value and othmask=value
Set the permission mask for ADFS ‘owner’ permissions and ‘other’
permissions, respectively (default: 0700 and 0077, respec‐
tively). See also /usr/src/linux/Documentation/filesys‐

Mount options for affs
uid=value and 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 and setgid=value
Set the owner and group of all files.

Set the mode of all files to value & 0777 disregarding the orig‐
inal permissions. Add search permission to directories that
have read permission. The value is given in octal.

Do not allow any changes to the protection bits on the filesys‐

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. Strange…

Print an informational message for each successful mount.

Prefix used before volume name, when following a link.

Prefix (of length at most 30) used before ‘/’ when following a
symbolic link.

(Default: 2.) Number of unused blocks at the start of the

Give explicitly the location of the root block.

Give blocksize. Allowed values are 512, 1024, 2048, 4096.

These options are accepted but ignored. (However, quota utili‐
ties may react to such strings in /etc/fstab.)

Mount options for btrfs
Btrfs is a copy-on-write filesystem for Linux aimed at implementing
advanced features while focusing on fault tolerance, repair, and easy

Debugging option to force all block allocations above a certain
byte threshold on each block device. The value is specified in
bytes, optionally with a K, M, or G suffix, case insensitive.
Default is 1MB.

Disable/enable auto defragmentation. Auto defragmentation
detects small random writes into files and queues them up for
the defrag process. Works best for small files; not well-suited
for large database workloads.

These debugging options control the behavior of the integrity
checking module(the BTRFS_FS_CHECK_INTEGRITY config option

check_int enables the integrity checker module, which examines
all block-write requests to ensure on-disk consistency, at a
large memory and CPU cost.

check_int_data includes extent data in the integrity checks, and
implies the check_int option.

check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_*
values as defined in fs/btrfs/check-integrity.c, to control the
integrity checker module behavior.

See comments at the top of fs/btrfs/check-integrity.c for more

Set the interval of periodic commit, 30 seconds by default.
Higher values defer data being synced to permanent storage, with
obvious consequences when the system crashes. The upper bound
is not forced, but a warning is printed if it’s more than 300
seconds (5 minutes).

Control BTRFS file data compression. Type may be specified as
“zlib” “lzo” or “no” (for no compression, used for remounting).
If no type is specified, zlib is used. If compress-force is
specified, all files will be compressed, whether or not they
compress well. If compression is enabled, nodatacow and nodata‐
sum are disabled.

Allow mounts to continue with missing devices. A read-write
mount may fail with too many devices missing, for example if a
stripe member is completely missing.

Specify a device during mount so that ioctls on the control
device can be avoided. Especially useful when trying to mount a
multi-device setup as root. May be specified multiple times for
multiple devices.

Disable/enable the discard mount option. The discard function
issues frequent commands to let the block device reclaim space
freed by the filesystem. This is useful for SSD devices, thinly
provisioned LUNs and virtual machine images, but may have a sig‐
nificant performance impact. (The fstrim command is also avail‐
able to initiate batch trims from userspace.)

Disable/enable debugging option to be more verbose in some
ENOSPC conditions.

Action to take when encountering a fatal error:
“bug” – BUG() on a fatal error. This is the default.
“panic” – panic() on a fatal error.

The flushoncommit mount option forces any data dirtied by a
write in a prior transaction to commit as part of the current
commit. This makes the committed state a fully consistent view
of the filesystem from the application’s perspective (i.e., it
includes all completed filesystem operations). This was previ‐
ously the behavior only when a snapshot is created.

Enable free inode number caching. Defaults to off due to an
overflow problem when the free space CRCs don’t fit inside a
single page.

Specify the maximum amount of space, in bytes, that can be
inlined in a metadata B-tree leaf. The value is specified in
bytes, optionally with a K, M, or G suffix, case insensitive.
In practice, this value is limited by the root sector size, with
some space unavailable due to leaf headers. For a 4k sector‐
size, max inline data is ~3900 bytes.

Specify that 1 metadata chunk should be allocated after every
value data chunks. Off by default.

noacl Enable/disable support for Posix Access Control Lists (ACLs).
See the acl(5) manual page for more information about ACLs.

Enable/disable the use of block-layer write barriers. Write
barriers ensure that certain IOs make it through the device
cache and are on persistent storage. If disabled on a device
with a volatile (non-battery-backed) write-back cache, the
nobarrier option will lead to filesystem corruption on a system
crash or power loss.

Enable/disable data copy-on-write for newly created files. This
option implies nodatasum, and disables all compression.

Enable/disable data checksumming for newly created files. This
option implies datacow.

Enable/disable the tree logging used for fsync and O_SYNC

Enable autorecovery attempts if a bad tree root is found at
mount time. Currently this scans a list of several previous
tree roots and tries to use the first readable.

Force check and rebuild procedure of the UUID tree. This should
not normally be needed.

Skip automatic resume of an interrupted balance operation after
mount. May be resumed with “btrfs balance resume.”

Disable freespace cache loading without clearing the cache.

Force clearing and rebuilding of the disk space cache if some‐
thing has gone wrong.

Options to control ssd allocation schemes. By default, BTRFS
will enable or disable ssd allocation heuristics depending on
whether a rotational or nonrotational disk is in use. The ssd
and nossd options can override this autodetection.

The ssd_spread mount option attempts to allocate into big chunks
of unused space, and may perform better on low-end ssds.
ssd_spread implies ssd, enabling all other ssd heuristics as

Mount subvolume at path rather than the root subvolume. The
path is relative to the top level subvolume.

Mount subvolume specified by an ID number rather than the root
subvolume. This allows mounting of subvolumes which are not in
the root of the mounted filesystem. You can use “btrfs subvol‐
ume list” to see subvolume ID numbers.

subvolrootid=objectid (deprecated)
Mount subvolume specified by objectid rather than the root sub‐
volume. This allows mounting of subvolumes which are not in the
root of the mounted filesystem. You can use “btrfs subvolume
show ” to see the object ID for a subvolume.

The number of worker threads to allocate. The default number is
equal to the number of CPUs + 2, or 8, whichever is smaller.

Allow subvolumes to be deleted by a non-root user. Use with

Mount options for cifs
See the options section of the mount.cifs(8) man page (cifs-utils pack‐
age must be installed).

Mount options for coherent

Mount options for debugfs
The debugfs filesystem is a pseudo filesystem, traditionally mounted on
/sys/kernel/debug. As of kernel version 3.4, debugfs has the following

uid=n, gid=n
Set the owner and group of the mountpoint.

Sets the mode of the mountpoint.

Mount options for devpts
The devpts filesystem is a pseudo filesystem, traditionally mounted on
/dev/pts. In order 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 slave can be accessed as

uid=value and gid=value
This sets the owner or the group of newly created PTYs to the
specified values. When nothing is specified, they will be set
to the UID and GID of the creating process. For example, if
there is a tty group with GID 5, then gid=5 will cause newly
created PTYs to belong to the tty group.

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.

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 CON‐
FIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configu‐

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.


Set the mode for the new ptmx device node in the devpts filesys‐

With the support for multiple instances of devpts (see newin‐
stance option above), each instance has a private ptmx node in
the root of the devpts filesystem (typically /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 start‐
ing with 2.6.29. Further, this option is valid only if CON‐
FIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configu‐

Mount options for ext2
The `ext2′ filesystem is the standard Linux filesystem. Since Linux
2.5.46, for most mount options the default is determined by the
filesystem superblock. Set them with tune2fs(8).

Support POSIX Access Control Lists (or not).

Set the behavior for the statfs system call. The minixdf behav‐
ior 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 or nocheck
No checking is done at mount time. This is the default. This
is fast. It is wise to invoke e2fsck(8) every now and then,
e.g. at boot time. The non-default behavior is unsupported
(check=normal and check=strict options have been removed). Note
that these mount options don’t have to be supported if ext4 ker‐
nel driver is used for ext2 and ext3 filesystems.

debug Print debugging info upon each (re)mount.

Define the behavior when an error is encountered. (Either
ignore errors and just mark the filesystem erroneous and con‐
tinue, 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(8).

grpid|bsdgroups and 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 in
which it is created; otherwise (the default) it takes the fsgid
of the current process, unless the directory has the setgid bit
set, in which case it takes the gid from the parent directory,
and also gets the setgid bit set if it is a directory itself.

The usrquota (same as quota) mount option enables user quota
support on the filesystem. grpquota enables group quotas sup‐
port. You need the quota utilities to actually enable and man‐
age the quota system.

Disables 32-bit UIDs and GIDs. This is for interoperability
with older kernels which only store and expect 16-bit values.

oldalloc or orlov
Use old allocator or Orlov allocator for new inodes. Orlov is

resgid=n and resuid=n
The ext2 filesystem reserves a certain percentage of the avail‐
able space (by default 5%, see mke2fs(8) and tune2fs(8)). These
options determine who can use the reserved blocks. (Roughly:
whoever has the specified uid, or belongs to the specified

sb=n Instead of block 1, use block n as superblock. This could be
useful when the filesystem has been damaged. (Earlier, copies
of the superblock would be made every 8192 blocks: in block 1,
8193, 16385, … (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 1 k
units. Thus, if you want to use logical block 32768 on a
filesystem with 4 k blocks, use “sb=131072”.

Support “user.” extended attributes (or not).

Mount options for ext3
The ext3 filesystem is a version of the ext2 filesystem which has been
enhanced with journaling. It supports the same options as ext2 as well
as the following additions:

Update the ext3 filesystem’s journal to the current format.

When a journal already exists, this option is ignored. Other‐
wise, it specifies the number of the inode which will represent
the ext3 filesystem’s journal file; ext3 will create a new jour‐
nal, overwriting the old contents of the file whose inode number
is inum.

When the external journal device’s major/minor numbers have
changed, these options allow the user to specify the new journal
location. The journal device is identified either through its
new major/minor numbers encoded in devnum, or via a path to the

Don’t load the journal on mounting. Note that if the filesystem
was not unmounted cleanly, skipping the journal replay will lead
to the filesystem containing inconsistencies that can lead to
any number of problems.

Specifies the journaling mode for file data. Metadata is always
journaled. To use modes other than ordered on the root filesys‐
tem, pass the mode to the kernel as boot parameter, e.g. root‐

All data is committed into the journal prior to being
written into the main filesystem.

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.

Data ordering is not preserved – data may be written into
the main filesystem after its metadata has been 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.

Just print an error message if an error occurs in a file data
buffer in ordered mode.

Abort the journal if an error occurs in a file data buffer in
ordered mode.

barrier=0 / barrier=1
This disables / enables the use of write barriers in the jbd
code. barrier=0 disables, barrier=1 enables (default). This
also requires an IO stack which can support barriers, and if jbd
gets an error on a barrier write, it will disable barriers 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.

Sync all data and metadata every nrsec seconds. The default
value is 5 seconds. Zero means default.

Enable Extended User Attributes. See the attr(5) manual page.

acl Enable POSIX Access Control Lists. See the acl(5) manual page.

Apart from the old quota system (as in ext2, jqfmt=vfsold aka
version 1 quota) ext3 also supports journaled quotas (version 2
quota). jqfmt=vfsv0 enables journaled quotas. For journaled
quotas the mount options usrjquota=aquota.user and are required to tell the quota system
which quota database files to use. Journaled quotas have the
advantage that even after a crash no quota check is required.

Mount options for 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, norecovery, 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, usrquota usrjquota, grpjquota and jqfmt are
backwardly compatible with ext3 or ext2.

Enable checksumming of the journal transactions. This will
allow the recovery code in e2fsck and the kernel to detect cor‐
ruption in the kernel. It is a compatible change and will be
ignored by older kernels.

Commit block can be written to disk without waiting for descrip‐
tor blocks. If enabled, older kernels cannot mount the device.
This will enable ‘journal_checksum’ internally.

barrier=0 / barrier=1 / barrier / nobarrier
These mount options have the same effect as in ext3. The mount
options “barrier” and “nobarrier” are added for consistency with
other ext4 mount options.

The ext4 filesystem enables write barriers by default.

This tuning parameter controls the maximum number of inode table
blocks that ext4’s inode table readahead algorithm will pre-read
into the buffer cache. The value must be a power of 2. The
default value is 32 blocks.

Number of filesystem blocks that mballoc will try to use for
allocation size and alignment. For RAID5/6 systems this should
be the number of data disks * RAID chunk size in filesystem

Deferring block allocation until write-out time.

Disable delayed allocation. Blocks are allocated when data is
copied from user to page cache.

Maximum amount of time ext4 should wait for additional filesys‐
tem 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 can be 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 auto‐
matically 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 has been running is less than the commit
time, ext4 will try sleeping for the commit time to see if other
operations will join the transaction. The commit time is capped
by the max_batch_time, which defaults to 15000 µs (15 ms). This
optimization can be turned off entirely by setting
max_batch_time to 0.

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.

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 pur‐
poses. This is normally used while remounting a filesystem
which is already mounted.

Many broken applications don’t use fsync() when replacing exist‐
ing files via patterns such as

fd = open(“”)/write(fd,…)/close(fd)/ rename(“”,

or worse yet

fd = open(“foo”, O_TRUNC)/write(fd,…)/close(fd).

If auto_da_alloc is enabled, ext4 will detect 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
can happen when a system crashes before the delayed allocation
blocks are forced to disk.

Do not initialize any uninitialized inode table blocks in the
background. This feature may be used by installation CD’s so
that the install process can complete as quickly as possible;
the inode table initialization process would then be deferred
until the next time the filesystem is mounted.

The lazy itable init code will wait n times the number of mil‐
liseconds it took to zero out the previous block group’s inode
table. This minimizes the impact on system performance while
the filesystem’s inode table is being initialized.

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 has been done.

Disables 32-bit UIDs and GIDs. This is for interoperability
with older kernels which only store and expect 16-bit values.

This options allows to enables/disables the in-kernel facility
for tracking filesystem metadata blocks within internal data
structures. This allows multi-block allocator and other rou‐
tines to quickly locate extents which might overlap with
filesystem metadata blocks. This option is intended for debug‐
ging purposes and since it negatively affects the performance,
it is off by default.

Controls whether or not ext4 should use the DIO read locking.
If the dioread_nolock option is specified ext4 will allocate
uninitialized extent before buffer write and convert the extent
to initialized after IO completes. This approach allows ext4
code to avoid using inode mutex, which improves scalability on
high speed storages. However this does not work with data jour‐
naling and dioread_nolock option will be 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).

This limits the size of the directories so that any attempt to
expand them beyond the specified limit in kilobytes will cause
an ENOSPC error. This is useful in memory-constrained environ‐
ments, where a very large directory can cause severe performance
problems or even provoke the Out Of Memory killer. (For example,
if there is only 512 MB memory available, a 176 MB directory may
seriously cramp the system’s style.)

Enable 64-bit inode version support. This option is off by

Mount options for fat
(Note: fat is not a separate filesystem, but a common part of the
msdos, umsdos and vfat filesystems.)

Set blocksize (default 512). This option is obsolete.

uid=value and gid=value
Set the owner and group of all files. (Default: the uid and gid
of the current process.)

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.

Set the umask applied to directories only. The default is the
umask of the current process. The value is given in octal.

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.

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 `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 lower case 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 that contain long parts or spe‐ cial characters that are sometimes used on Linux but are not accepted by MS-DOS (+, =, etc.) are rejected. codepage=value Sets the codepage for converting to shortname characters on FAT and VFAT filesystems. By default, codepage 437 is used. conv=mode The fat filesystem can perform CRLF<-->NL conversion (MS-DOS
text format to UNIX text format) in the kernel. The following
conversion modes are available:

b[inary] No translation is performed. This is the default.

t[ext] CRLF<-->NL translation is performed on all files.

a[uto] CRLF<-->NL translation is performed on all files that
don’t have a “well-known binary” extension. The list of
known extensions can be found 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,

Programs that do computed lseeks won’t like in-kernel text con‐
version. Several people have had their data ruined by this
translation. Beware!

For filesystems mounted in binary mode, a conversion tool (from‐
dos/todos) is available. This option is obsolete.

Forces the driver to use the CVF (Compressed Volume File) module
cvf_module instead of auto-detection. If the kernel supports
kmod, the cvf_format=xxx option also controls on-demand CVF mod‐
ule loading. This option is obsolete.

Option passed to the CVF module. This option is obsolete.

debug Turn on the debug flag. A version string and a list of filesys‐
tem parameters will be printed (these data are also printed if
the parameters appear to be inconsistent).

If set, causes discard/TRIM commands to be issued to the block
device when blocks are freed. This is useful for SSD devices
and sparse/thinly-provisioned LUNs.

If set, use a fallback default BIOS Parameter Block configura‐
tion, determined by backing device size. These static parameters
match defaults assumed by DOS 1.x for 160 kiB, 180 kiB, 320 kiB,
and 360 kiB floppies and floppy images.

Specify FAT behavior on critical errors: panic, continue without
doing anything, or remount the partition in read-only mode
(default behavior).

Specify a 12, 16 or 32 bit fat. This overrides the automatic
FAT type detection routine. Use with caution!

Character set to use for converting between 8 bit characters and
16 bit Unicode characters. The default is iso8859-1. Long
filenames are stored on disk in Unicode format.

Enable this only if you want to export the FAT filesystem over

stale_rw: This option maintains an index (cache) of directory
inodes which is used by the nfs-related code to improve look-
ups. Full file operations (read/write) over NFS are supported
but with cache eviction at NFS server, this could result in spu‐
rious ESTALE errors.

nostale_ro: This option bases the inode number and filehandle on
the on-disk location of a file in the FAT directory entry. This
ensures that ESTALE will not be returned after a file is evicted
from the inode cache. However, it means that operations such as
rename, create and unlink could cause filehandles that previ‐
ously pointed at one file to point at a different file, poten‐
tially causing data corruption. For this reason, this option
also mounts the filesystem readonly.

To maintain backward compatibility, ‘-o nfs’ is also accepted,
defaulting to stale_rw.

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 in order to avoid the
pitfalls of local time.

Set offset for conversion of timestamps from local time used by
FAT to UTC. I.e., minutes minutes will be subtracted from each
timestamp to convert it to UTC used internally by Linux. This is
useful when the time zone set in the kernel via settimeofday(2)
is not the time zone used by the filesystem. Note that this
option still does not provide correct time stamps in all cases
in presence of DST – time stamps in a different DST setting will
be off by one hour.

quiet Turn on the quiet flag. Attempts to chown or chmod files do not
return errors, although they fail. Use with caution!

rodir FAT has the ATTR_RO (read-only) attribute. On Windows, the
ATTR_RO of the directory will just be ignored, and is used only
by applications as a flag (e.g. it’s set for the customized

If you want to use ATTR_RO as read-only flag even for the direc‐
tory, set this option.

If set, the execute permission bits of the file will be allowed
only if the extension part of the name is .EXE, .COM, or .BAT.
Not set by default.

If set, ATTR_SYS attribute on FAT is handled as IMMUTABLE flag
on Linux. Not set by default.

flush If set, the filesystem will try to flush to disk more early than
normal. Not set by default.

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 conventions onto
a FAT filesystem.

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 values: ‘????’.

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

Select the CDROM session to mount. Defaults to leaving that
decision to the CDROM driver. This option will fail with any‐
thing but a CDROM as underlying device.

part=n Select partition number n from the device. Only makes sense for
CDROMs. Defaults to not parsing the partition table at all.

quiet Don’t complain about invalid mount options.

Mount options for hpfs
uid=value and gid=value
Set the owner and group of all files. (Default: the uid and gid
of the current process.)

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.

Convert all files names to lower case, or leave them. (Default:

For conv=text, delete some random CRs (in particular, all fol‐
lowed by NL) when reading a file. For conv=auto, choose more or
less at random between conv=binary and conv=text. For
conv=binary, just read what is in the file. This is the

Do not abort mounting when certain consistency checks fail.

Mount options for iso9660
ISO 9660 is a standard describing a filesystem structure to be used on
CD-ROMs. (This filesystem type is also seen on some DVDs. See also the
udf filesystem.)

Normal iso9660 filenames appear in a 8.3 format (i.e., DOS-like
restrictions on filename length), and in addition all characters are in
upper case. 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 of these UNIX-
like features. Basically there are extensions to each directory record
that supply all of the additional information, and when Rock Ridge is
in use, the filesystem is indistinguishable from a normal UNIX filesys‐
tem (except that it is read-only, of course).

norock Disable the use of Rock Ridge extensions, even if available.
Cf. map.

Disable the use of Microsoft Joliet extensions, even if avail‐
able. Cf. map.

With check=relaxed, a filename is first converted to lower case
before doing the lookup. This is probably only meaningful
together with norock and map=normal. (Default: check=strict.)

uid=value and 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.)

For non-Rock Ridge volumes, normal name translation maps upper
to lower case ASCII, drops a trailing `;1′, and converts `;’ to
`.’. With map=off no name translation is done. See norock.
(Default: map=normal.) map=acorn is like map=normal but also
apply Acorn extensions if present.

For non-Rock Ridge volumes, give all files the indicated mode.
(Default: read and execute permission for everybody.) Since
Linux 2.1.37 one no longer needs 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 filenames, this
may make the ordinary files inaccessible.)

Set the block size to the indicated value. (Default:

(Default: conv=binary.) Since Linux 1.3.54 this option has no
effect anymore. (And non-binary settings used to be very dan‐
gerous, 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.

Select number of session on multisession CD. (Since 2.3.4.)

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 exten‐

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.

Mount options for jfs
Character set to use for converting from Unicode to ASCII. The
default is to do no conversion. Use iocharset=utf8 for UTF8
translations. This requires CONFIG_NLS_UTF8 to be set in the
kernel .config file.

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 key‐
word with no value will grow the volume to the full size of the

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 ends.

Default. Commit metadata changes to the journal. Use this
option to remount a volume where the nointegrity option was pre‐
viously specified in order to restore normal behavior.

Define the behavior when an error is encountered. (Either
ignore errors and just mark the filesystem erroneous and con‐
tinue, or remount the filesystem read-only, or panic and halt
the system.)

These options are accepted but ignored.

Mount options for minix

Mount options for msdos
See mount options for fat. If the msdos filesystem detects an incon‐
sistency, it reports an error and sets the file system read-only. The
filesystem can be made writable again by remounting it.

Mount options for ncpfs
Just like nfs, the ncpfs implementation expects a binary argument (a
struct ncp_mount_data) to the mount system call. This argument is con‐
structed by ncpmount(8) and the current version of mount (2.12) does
not know anything about ncpfs.

Mount options for nfs and nfs4
See the options section of the nfs(5) man page (nfs-utils package must
be installed).

The nfs and nfs4 implementation expects a binary argument (a struct
nfs_mount_data) to the mount system call. This argument is constructed
by mount.nfs(8) and the current version of mount (2.13) does not know
anything about nfs and nfs4.

Mount options for ntfs
Character set to use when returning file names. Unlike VFAT,
NTFS suppresses names that contain nonconvertible characters.

New name for the option earlier called iocharset.

utf8 Use UTF-8 for converting file names.

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 bigendian

posix=[0|1] If enabled (posix=1), the filesystem distinguishes between upper
and lower case. The 8.3 alias names are presented as hard links
instead of being suppressed. This option is obsolete.

uid=value, gid=value and 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.

Mount options for proc
uid=value and gid=value
These options are recognized, but have no effect as far as I can

Mount options for ramfs
Ramfs is a memory based filesystem. Mount it and you have it. Unmount
it and it is gone. Present since Linux 2.3.99pre4. There are no mount

Mount options for reiserfs
Reiserfs is a journaling filesystem.

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 will no longer be compatible with reiserfs 3.5

Choose which hash function reiserfs will use to find files
within directories.

A hash invented by Yury Yu. Rupasov. It is fast and pre‐
serves 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 implemented by Jeremy
Fitzhardinge. It 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 filesys‐

Tunes the block allocator. This may provide performance
improvements in some situations.

Tunes the block allocator. This may provide performance
improvements in some situations.

Disable the border allocator algorithm invented by Yury Yu.
Rupasov. This may provide performance improvements in some sit‐

nolog Disable journaling. This will provide 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 journaling operations, save for
actual writes into its journaling 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(8). This option is used to disable packing of files into
the tree.

Replay the transactions which are in the journal, but do not
actually mount the filesystem. Mainly used by reiserfsck.

A remount option which permits online expansion of reiserfs par‐
titions. Instructs reiserfs to assume that the device has num‐
ber blocks. This option is designed for use with devices which
are under logical volume management (LVM). There is a special
resizer utility which can be obtained from

Enable Extended User Attributes. See the attr(5) manual page.

acl Enable POSIX Access Control Lists. See the acl(5) manual page.

barrier=none / barrier=flush
This disables / enables the use of write barriers in the jour‐
naling code. barrier=none disables, barrier=flush enables
(default). This also requires an IO stack which can support
barriers, and if reiserfs gets an error on a barrier write, it
will disable barriers 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.

Mount options for romfs

Mount options for squashfs

Mount options for smbfs
Just like nfs, the smbfs implementation expects a binary argument (a
struct smb_mount_data) to the mount system call. This argument is con‐
structed by smbmount(8) and the current version of mount (2.12) does
not know anything about smbfs.

Mount options for sysv

Mount options for tmpfs
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%

The same as size, but in blocks of PAGE_CACHE_SIZE

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

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 (kibi), binary
mega (mebi) and binary giga (gibi)) and can be changed on remount.

mode= Set initial permissions of the root directory.

uid= The user id.

gid= The group id.

mpol=[default|prefer:Node|bind:NodeList|interleave|interleave:NodeList] Set the NUMA memory allocation policy for all files in that
instance (if the kernel CONFIG_NUMA is enabled) – which can be
adjusted on the fly via ‘mount -o remount …’

prefers to allocate memory from the local node

prefers to allocate memory from the given Node

allocates memory only from nodes in NodeList

prefers to allocate from each node in turn

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-minus”-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 will fail
if the running kernel does not support NUMA; and will fail if
its nodelist specifies a node which is not online. If your sys‐
tem 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 advis‐
able to omit the mpol option from automatic mount options. It
can be added later, when the tmpfs is already mounted on Mount‐
Point, by ‘mount -o remount,mpol=Policy:NodeList MountPoint’.

Mount options for 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



UBI device number X, volume with name




UBI device number 0, volume with name


Alternative ! separator may be used instead of :.

The following mount options are available:

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.

Do not bulk-read. This is the default.

Check data CRC-32 checksums. This is the default.

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.

Select the default compressor which is used when new files are
written. It is still possible to read compressed files if
mounted with the none option.

Mount options for 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.

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.

Show deleted files in lists.

Unset strict conformance.

Set the NLS character set.

bs= Set the block size. (May not work unless 2048.)

novrs Skip volume sequence recognition.

Set the CDROM session counting from 0. Default: last session.

Override standard anchor location. Default: 256.

Override the VolumeDesc location. (unused)

Override the PartitionDesc location. (unused)

Set the last block of the filesystem.

Override the fileset block location. (unused)

Override the root directory location. (unused)

Mount options for ufs
UFS is a filesystem widely used in different operating systems.
The problem are 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, read only.
(Don’t forget to give the -r option.)

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.

For filesystems created by NeXTStep (on NeXT station)
(currently read only).

For NextStep CDROMs (block_size == 2048), read-only.

For filesystems created by OpenStep (currently read
only). The same filesystem type is also used by Mac OS

Set behavior on 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.

Mount options for umsdos
See mount options for msdos. The dotsOK option is explicitly killed by

Mount options for vfat
First of all, the mount options for fat are recognized. The dotsOK
option is explicitly killed by vfat. Furthermore, there are

Translate unhandled Unicode characters to special escaped
sequences. This lets you backup and restore filenames 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 invalid on the vfat filesystem.
The escape sequence that gets used, where u is the Unicode char‐
acter, is: ‘:’, (u & 0x3f), ((u>>6) & 0x3f), (u>>12).

posix Allow two files with names that only differ in case. This
option is obsolete.

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 filenames which
fit into 8.3 characters. If a long name for a file exists, it
will always be the preferred one for display. There are four

lower Force the short name to lower case upon display; store a
long name when the short name is not all upper case.

win95 Force the short name to upper case upon display; store a
long name when the short name is not all upper case.

winnt Display the short name as is; store a long name when the
short name is not all lower case or all upper case.

mixed Display the short name as is; store a long name when the
short name is not all upper case. This mode is the
default since Linux 2.6.32.

Mount options for usbfs
devuid=uid and devgid=gid and 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 and busgid=gid and 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 and listgid=gid and 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.

Mount options for xenix

Mount options for xfs
See the options section of the xfs(5) man page (xfsprogs package must
be installed).

One further possible type is a mount via the loop device. For example,
the command

mount /tmp/disk.img /mnt -t vfat -o loop=/dev/loop3

will set 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 just an option `-o loop’
is given), then mount will try 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 three options, namely loop, offset and
sizelimit, that are really options to losetup(8). (These options can
be used in addition to those specific to the filesystem type.)

Since Linux 2.6.25 auto-destruction of loop devices is supported, mean‐
ing that any loop device allocated by mount will be freed by umount
independently of /etc/mtab.

You can also free a loop device by hand, using losetup -d or umount -d.

mount has the following return codes (the bits can be ORed):

0 success

1 incorrect invocation or permissions

2 system error (out of memory, cannot fork, no more loop devices)

4 internal mount bug

8 user interrupt

16 problems writing or locking /etc/mtab

32 mount failure

64 some mount succeeded

The command mount -a returns 0 (all succeeded), 32 (all failed), or 64
(some failed, some succeeded).

The syntax of external mount helpers is:

/sbin/mount.suffix spec dir [-sfnv] [-o options] [-t type.sub‐

where the suffix is the filesystem type and the -sfnvo options have the
same meaning as the normal mount options. The -t option is used for
filesystems with subtypes support (for example /sbin/mount.fuse -t

The command mount does not pass the mount options unbindable, runbind‐
able, private, rprivate, slave, rslave, shared, rshared, auto, noauto,
comment, x-*, loop, offset and sizelimit to the mount. helpers.
All other options are used in a comma-separated list as argument to the
-o option.

/etc/fstab filesystem table

/etc/mtab table of mounted filesystems

/etc/mtab~ lock file

/etc/mtab.tmp temporary file

/etc/filesystems a list of filesystem types to try

LIBMOUNT_FSTAB= overrides the default location of the fstab file (ignored for

LIBMOUNT_MTAB= overrides the default location of the mtab file (ignored for

enables libmount debug output

enables libblkid debug output

enables loop device setup debug output


mount, umount, fstab(5), umount(8), swapon(8), findmnt(8),
nfs(5), xfs(5), e2label(8), xfs_admin(8), mountd(8), nfsd(8),
mke2fs(8), tune2fs(8), losetup(8)


It is possible for a corrupted filesystem to cause a crash.

Some Linux filesystems don’t support -o sync and -o dirsync (the ext2,
ext3, fat and vfat filesystems do support synchronous updates (a la
BSD) when mounted with the sync option).

The -o remount may not be able to change mount parameters (all ext2fs-
specific parameters, except sb, are changeable with a remount, for
example, but you can’t change gid or umask for the fatfs).

It is possible that files /etc/mtab and /proc/mounts don’t match on
systems with regular mtab file. The first file is based only on the
mount command options, but the content of the second file also depends
on the kernel and others settings (e.g. remote NFS server. In particu‐
lar case the mount command may reports unreliable information about a
NFS mount point and the /proc/mounts file usually contains more reli‐
able information.) This is another reason to replace mtab file with
symlink to the /proc/mounts file.

Checking files on NFS filesystem referenced by file descriptors (i.e.
the fcntl and ioctl families of functions) may lead to inconsistent
result due to the lack of consistency check in kernel even if noac is

The loop option with the offset or sizelimit options used may fail when
using older kernels if the mount command can’t confirm that the size of
the block device has been configured as requested. This situation can
be worked around by using the losetup command manually before calling
mount with the configured loop device.

A mount command existed in Version 5 AT&T UNIX.

Karel Zak

The mount command is part of the util-linux package and is available

util-linux July 2014 MOUNT(8)