MOUNT(8) System Administration MOUNT(8)
NAME
mount - mount a filesystem
SYNOPSIS
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
DESCRIPTION
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 several 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 example:
mount --target /mountpoint
The listing.
The listing mode is maintained for backward compatibility only.
For more robust and customizable output use findmnt(8), especially in your scripts. Note that control characters in the mountpoint name are replaced with '?'.
The following command lists all mounted filesystems (of type 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 knuth.cwi.nl:/dir. It is also
possible to indicate a block special 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 (GPT).)
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 sys‐
tem.
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 portable. The mount(8) com‐
mand internally uses udev symlinks, so the use of symlinks in /etc/fstab has no advantage over tags. For more details see libblkid(3).
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 details).
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 simultaneously.
When mounting a filesystem mentioned in fstab or mtab, it suffices 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 filesystems 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 ones.
The mount program does not read the /etc/fstab file if both device (or LABEL, UUID, PARTUUID or PARTLABEL) and dir are specified. 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
or
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 implemented in userspace by additional remount mount(2) 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 mountpoint.
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 submounts 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 master, 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(2) 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(2) syscalls when the preceding 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
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.
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 command-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 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 an existing record in /etc/mtab and fails when the record already exists (with a
regular 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 reiserfs 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 example 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 _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
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 OPTIONS and FILESYSTEM-SPECIFIC MOUNT OPTIONS sections.
-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 example, 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(3); files that start with "." or without an .fstab extension are ignored. The option can be speci‐
fied more than once. This option is mostly designed for initramfs or chroot scripts where additional configuration is specified beyond standard system configuration.
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/modules/$(uname -r)/kernel/fs
for a complete list of the filesystems. 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 'sshfs#example.com' 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 famil‐
iar, 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 specify the filesystem types on
which no action should be taken. The prefix no has no effect when specified in an /etc/fstab entry.
The prefix no can be meaningful with the -a option. For example, 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(2) system call, and no detailed knowledge of the filesystem 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 program. In order to make it possible to treat all types in a uniform way, mount will execute the program
/sbin/mount.type (if that exists) when called with type type. Since different 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.
-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 synonym is -o rw.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
FILESYSTEM-INDEPENDENT MOUNT OPTIONS
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 filesystems).
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 controlled by kernel defaults. See also the descriptions of the strictatime and relatime 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). This works for all inode types (directories too), so implies nodiratime.
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 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.
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=, 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 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 a FS being mounted before that FS or inode becomes visible 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 context.
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 \
'context="system_u:object_r:tmp_t:s0:c127,c456",noexec'
For more details, see selinux(8).
defaults
Use the default options: rw, suid, dev, exec, auto, nouser, and async.
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 system.
diratime
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.
nodiratime
Do not update directory inode access times on this filesystem. If noatime option is set, this option is not needed.
dirsync
All directory updates within 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 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 subsequent options, as in the option
line group,dev,suid).
iversion
Every time the inode is modified, the i_version field will be incremented.
noiversion
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.
_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 has been 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 it doesn't break mutt
or other applications that need to know if a file has been 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. In addition, since Linux 2.6.30,
the file's last access time is always updated if it is more than 1 day old.
norelatime
Do not use the relatime feature. See also the strictatime mount option.
strictatime
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.
nostrictatime
Use the kernel's default behavior for inode access time updates.
lazytime
Only update times (atime, mtime, ctime) on the in-memory version of the file inode.
This mount option significantly reduces writes to the inode table for workloads that perform frequent random writes to preallocated 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(2), syncfs(2), or sync(2)
- an undeleted inode is evicted from memory
- more than 24 hours have passed since the i-node was written to disk.
nolazytime
Do not use the lazytime feature.
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.
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).
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 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 command.
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 user,exec,dev,suid).
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 subsequent 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(2) system
call. The suggested format is x-appname.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(2) in octal notation. The default mode is 0755. This functionality is supported only for
root users.
FILESYSTEM-SPECIFIC MOUNT OPTIONS
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 Documentation/filesystems.
Mount options for adfs
uid=value and gid=value
Set the owner and group of the files in the filesystem (default: uid=gid=0).
ownmask=value and 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.
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.
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. Strange...
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.)
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 administration.
alloc_start=bytes
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.
autodefrag
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.
check_int|check_int_data|check_int_print_mask=value
These debugging options control the behavior of the integrity checking module(the BTRFS_FS_CHECK_INTEGRITY config option required).
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 info.
commit=seconds
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).
compress|compress=type|compress-force|compress-force=type
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 nodatasum are disabled.
degraded
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.
device=devicepath
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.
discard
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 vir‐
tual machine images, but may have a significant performance impact. (The fstrim command is also available to initiate batch trims from userspace.)
enospc_debug
Disable/enable debugging option to be more verbose in some ENOSPC conditions.
fatal_errors=action
Action to take when encountering a fatal error:
"bug" - BUG() on a fatal error. This is the default.
"panic" - panic() on a fatal error.
flushoncommit
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 previously the behavior only when a snapshot is created.
inode_cache
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.
max_inline=bytes
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 lim‐
ited by the root sector size, with some space unavailable due to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
metadata_ratio=value
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.
nobarrier
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.
nodatacow
Enable/disable data copy-on-write for newly created files. This option implies nodatasum, and disables all compression.
nodatasum
Enable/disable data checksumming for newly created files. This option implies datacow.
notreelog
Enable/disable the tree logging used for fsync and O_SYNC writes.
recovery
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.
rescan_uuid_tree
Force check and rebuild procedure of the UUID tree. This should not normally be needed.
skip_balance
Skip automatic resume of an interrupted balance operation after mount. May be resumed with "btrfs balance resume."
nospace_cache
Disable freespace cache loading without clearing the cache.
clear_cache
Force clearing and rebuilding of the disk space cache if something has gone wrong.
ssd|nossd|ssd_spread
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 well.
subvol=path
Mount subvolume at path rather than the root subvolume. The path is relative to the top level subvolume.
subvolid=ID
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 subvolume list" to see subvolume
ID numbers.
subvolrootid=objectid (deprecated)
Mount subvolume specified by objectid rather than the root subvolume. 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.
thread_pool=number
The number of worker threads to allocate. The default number is equal to the number of CPUs + 2, or 8, whichever is smaller.
user_subvol_rm_allowed
Allow subvolumes to be deleted by a non-root user. Use with caution.
Mount options for cifs
See the options section of the mount.cifs(8) man page (cifs-utils package must be installed).
Mount options for coherent
None.
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 options:
uid=n, gid=n
Set the owner and group of the mountpoint.
mode=value
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 /dev/pts/.
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.
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 (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 starting with 2.6.29. Further, this option is valid only if CONFIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configuration.
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).
acl|noacl
Support POSIX Access Control Lists (or 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 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 kernel driver is used for ext2 and ext3 filesystems.
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 just 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(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.
grpquota|noquota|quota|usrquota
The usrquota (same as quota) mount option enables user quota support on the filesystem. grpquota enables group quotas support. You need the quota utilities to actually enable and manage the quota system.
nouid32
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 default.
resgid=n and resuid=n
The ext2 filesystem reserves a certain percentage of the available 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 group.)
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 f