TUNE2FS(8) System Manager’s Manual TUNE2FS(8)


tune2fs – adjust tunable filesystem parameters on ext2/ext3/ext4 filesystems


tune2fs [ -l ] [ -c max-mount-counts ] [ -e errors-behavior ] [ -f ] [ -i interval-between-checks ] [ -j ] [

-J journal-options ] [ -m reserved-blocks-percentage ] [ -o [^]mount-options[,…] ] [ -r reserved-blocks-

count ] [ -s sparse-super-flag ] [ -u user ] [ -g group ] [ -C mount-count ] [ -E extended-options ] [ -L

volume-name ] [ -M last-mounted-directory ] [ -O [^]feature[,…] ] [ -Q quota-options ] [ -T time-last-

checked ] [ -U UUID ] device


tune2fs allows the system administrator to adjust various tunable filesystem parameters on Linux ext2, ext3,

or ext4 filesystems. The current values of these options can be displayed by using the -l option to

tune2fs(8) program, or by using the dumpe2fs(8) program.

The device specifier can either be a filename (i.e., /dev/sda1), or a LABEL or UUID specifier: “LABEL=volume-

name” or “UUID=uuid”. (i.e., LABEL=home or UUID=e40486c6-84d5-4f2f-b99c-032281799c9d).


-c max-mount-counts
Adjust the number of mounts after which the filesystem will be checked by e2fsck(8). If max-mount-

counts is 0 or -1, the number of times the filesystem is mounted will be disregarded by e2fsck(8) and

the kernel.

Staggering the mount-counts at which filesystems are forcibly checked will avoid all filesystems being

checked at one time when using journaled filesystems.

You should strongly consider the consequences of disabling mount-count-dependent checking entirely.

Bad disk drives, cables, memory, and kernel bugs could all corrupt a filesystem without marking the

filesystem dirty or in error. If you are using journaling on your filesystem, your filesystem will

never be marked dirty, so it will not normally be checked. A filesystem error detected by the kernel

will still force an fsck on the next reboot, but it may already be too late to prevent data loss at

that point.

See also the -i option for time-dependent checking.

-C mount-count
Set the number of times the filesystem has been mounted. If set to a greater value than the max-

mount-counts parameter set by the -c option, e2fsck(8) will check the filesystem at the next reboot.

-e error-behavior
Change the behavior of the kernel code when errors are detected. In all cases, a filesystem error

will cause e2fsck(8) to check the filesystem on the next boot. error-behavior can be one of the fol‐


continue Continue normal execution.

remount-ro Remount filesystem read-only.

panic Cause a kernel panic.

-E extended-options
Set extended options for the filesystem. Extended options are comma separated, and may take an argu‐

ment using the equals (‘=’) sign. The following extended options are supported:

Reset the MMP block (if any) back to the clean state. Use only if absolutely certain the

device is not currently mounted or being fscked, or major filesystem corruption can

result. Needs ‘-f’.

Adjust the initial MMP update interval to interval seconds. Specifying an interval of 0

means to use the default interval. The specified interval must be less than 300 seconds.

Requires that the mmp feature be enabled.

Configure the filesystem for a RAID array with stride-size filesystem blocks. This is the

number of blocks read or written to disk before moving to next disk. This mostly affects

placement of filesystem metadata like bitmaps at mke2fs(2) time to avoid placing them on a

single disk, which can hurt the performance. It may also be used by block allocator.

Configure the filesystem for a RAID array with stripe-width filesystem blocks per stripe.

This is typically be stride-size * N, where N is the number of data disks in the RAID

(e.g. RAID 5 N+1, RAID 6 N+2). This allows the block allocator to prevent read-modify-

write of the parity in a RAID stripe if possible when the data is written.

Set the default hash algorithm used for filesystems with hashed b-tree directories. Valid

algorithms accepted are: legacy, half_md4, and tea.

Set a set of default mount options which will be used when the file system is mounted.

Unlike the bitmask-based default mount options which can be specified with the -o option,

mount_option_string is an arbitrary string with a maximum length of 63 bytes, which is

stored in the superblock.

The ext4 file system driver will first apply the bitmask-based default options, and then

parse the mount_option_string, before parsing the mount options passed from the mount(8)


This superblock setting is only honored in 2.6.35+ kernels; and not at all by the ext2 and

ext3 file system drivers.

Set a flag in the filesystem superblock indicating that it may be mounted using experimen‐

tal kernel code, such as the ext4dev filesystem.

Clear the test_fs flag, indicating the filesystem should only be mounted using production-

level filesystem code.

-f Force the tune2fs operation to complete even in the face of errors. This option is useful when remov‐
ing the has_journal filesystem feature from a filesystem which has an external journal (or is cor‐

rupted such that it appears to have an external journal), but that external journal is not available.

If the filesystem appears to require journal replay, the -f flag must be specified twice to proceed.

WARNING: Removing an external journal from a filesystem which was not cleanly unmounted without first

replaying the external journal can result in severe data loss and filesystem corruption.

-g group
Set the group which can use the reserved filesystem blocks. The group parameter can be a numerical

gid or a group name. If a group name is given, it is converted to a numerical gid before it is stored

in the superblock.

-i interval-between-checks[d|m|w]
Adjust the maximal time between two filesystem checks. No suffix or d will interpret the number

interval-between-checks as days, m as months, and w as weeks. A value of zero will disable the time-

dependent checking.

It is strongly recommended that either -c (mount-count-dependent) or -i (time-dependent) checking be

enabled to force periodic full e2fsck(8) checking of the filesystem. Failure to do so may lead to

filesystem corruption (due to bad disks, cables, memory, or kernel bugs) going unnoticed, ultimately

resulting in data loss or corruption.

-j Add an ext3 journal to the filesystem. If the -J option is not specified, the default journal parame‐
ters will be used to create an appropriately sized journal (given the size of the filesystem) stored

within the filesystem. Note that you must be using a kernel which has ext3 support in order to actu‐

ally make use of the journal.

If this option is used to create a journal on a mounted filesystem, an immutable file, .journal, will

be created in the top-level directory of the filesystem, as it is the only safe way to create the

journal inode while the filesystem is mounted. While the ext3 journal is visible, it is not safe to

delete it, or modify it while the filesystem is mounted; for this reason the file is marked immutable.

While checking unmounted filesystems, e2fsck(8) will automatically move .journal files to the invisi‐

ble, reserved journal inode. For all filesystems except for the root filesystem, this should happen

automatically and naturally during the next reboot cycle. Since the root filesystem is mounted read-

only, e2fsck(8) must be run from a rescue floppy in order to effect this transition.

On some distributions, such as Debian, if an initial ramdisk is used, the initrd scripts will automat‐

ically convert an ext2 root filesystem to ext3 if the /etc/fstab file specifies the ext3 filesystem

for the root filesystem in order to avoid requiring the use of a rescue floppy to add an ext3 journal

to the root filesystem.

-J journal-options
Override the default ext3 journal parameters. Journal options are comma separated, and may take an

argument using the equals (‘=’) sign. The following journal options are supported:

Create a journal stored in the filesystem of size journal-size megabytes. The size of

the journal must be at least 1024 filesystem blocks (i.e., 1MB if using 1k blocks, 4MB if

using 4k blocks, etc.) and may be no more than 102,400 filesystem blocks. There must be

enough free space in the filesystem to create a journal of that size.

Specify the location of the journal. The argument journal-location can either be speci‐

fied as a block number, or if the number has a units suffix (e.g., ‘M’, ‘G’, etc.) inter‐

pret it as the offset from the beginning of the file system.

Attach the filesystem to the journal block device located on external-journal. The exter‐

nal journal must have been already created using the command

mke2fs -O journal_dev external-journal

Note that external-journal must be formatted with the same block size as filesystems which

will be using it. In addition, while there is support for attaching multiple filesystems

to a single external journal, the Linux kernel and e2fsck(8) do not currently support

shared external journals yet.

Instead of specifying a device name directly, external-journal can also be specified by

either LABEL=label or UUID=UUID to locate the external journal by either the volume label

or UUID stored in the ext2 superblock at the start of the journal. Use dumpe2fs(8) to

display a journal device’s volume label and UUID. See also the -L option of tune2fs(8).

Only one of the size or device options can be given for a filesystem.

-l List the contents of the filesystem superblock, including the current values of the parameters that
can be set via this program.

-L volume-label
Set the volume label of the filesystem. Ext2 filesystem labels can be at most 16 characters long; if

volume-label is longer than 16 characters, tune2fs will truncate it and print a warning. The volume

label can be used by mount(8), fsck(8), and /etc/fstab(5) (and possibly others) by specifying

LABEL=volume_label instead of a block special device name like /dev/hda5.

-m reserved-blocks-percentage
Set the percentage of the filesystem which may only be allocated by privileged processes. Reserving

some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmen‐

tation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-

privileged processes are prevented from writing to the filesystem. Normally, the default percentage

of reserved blocks is 5%.

-M last-mounted-directory
Set the last-mounted directory for the filesystem.

-o [^]mount-option[,…]
Set or clear the indicated default mount options in the filesystem. Default mount options can be

overridden by mount options specified either in /etc/fstab(5) or on the command line arguments to

mount(8). Older kernels may not support this feature; in particular, kernels which predate 2.4.20

will almost certainly ignore the default mount options field in the superblock.

More than one mount option can be cleared or set by separating features with commas. Mount options

prefixed with a caret character (‘^’) will be cleared in the filesystem’s superblock; mount options

without a prefix character or prefixed with a plus character (‘+’) will be added to the filesystem.

The following mount options can be set or cleared using tune2fs:

debug Enable debugging code for this filesystem.

Emulate BSD behavior when creating new files: they will take the group-id of the directory

in which they were created. The standard System V behavior is the default, where newly

created files take on the fsgid of the current process, unless the directory has the set‐

gid 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.

Enable user-specified extended attributes.

acl Enable Posix Access Control Lists.

uid16 Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only
store and expect 16-bit values.

When the filesystem is mounted with journalling enabled, all data (not just metadata) is

committed into the journal prior to being written into the main filesystem.

When the filesystem is mounted with journalling enabled, all data is forced directly out

to the main file system prior to its metadata being committed to the journal.

When the filesystem is mounted with journalling enabled, data may be written into the main

filesystem after its metadata has been committed to the journal. This may increase

throughput, however, it may allow old data to appear in files after a crash and journal


The file system will be mounted with barrier operations in the journal disabled. (This

option is currently only supported by the ext4 file system driver in 2.6.35+ kernels.)

The file system will be mounted with the block_validity option enabled, which causes extra

checks to be performed after reading or writing from the file system. This prevents cor‐

rupted metadata blocks from causing file system damage by overwriting parts of the inode

table or block group descriptors. This comes at the cost of increased memory and CPU

overhead, so it is enabled only for debugging purposes. (This option is currently only

supported by the ext4 file system driver in 2.6.35+ kernels.)

The file system will be mounted with the discard mount option. This will cause the file

system driver to attempt to use the trim/discard feature of some storage devices (such as

SSD’s and thin-provisioned drives available in some enterprise storage arrays) to inform

the storage device that blocks belonging to deleted files can be reused for other pur‐

poses. (This option is currently only supported by the ext4 file system driver in 2.6.35+


The file system will be mounted with the nodelalloc mount option. This will disable the

delayed allocation feature. (This option is currently only supported by the ext4 file

system driver in 2.6.35+ kernels.)

-O [^]feature[,…]
Set or clear the indicated filesystem features (options) in the filesystem. More than one filesystem

feature can be cleared or set by separating features with commas. Filesystem features prefixed with a

caret character (‘^’) will be cleared in the filesystem’s superblock; filesystem features without a

prefix character or prefixed with a plus character (‘+’) will be added to the filesystem. For a

detailed description of the file system features, please see the man page ext4(5).

The following filesystem features can be set or cleared using tune2fs:

Use hashed b-trees to speed up lookups for large directories.

Allow more than 65000 subdirectories per directory.

extent Enable the use of extent trees to store the location of data blocks in inodes.

Enable the extended inode fields used by ext4.

Store file type information in directory entries.

Allow bitmaps and inode tables for a block group to be placed anywhere on the storage

media. Tune2fs will not reorganize the location of the inode tables and allocation bit‐

maps, as mke2fs(8) will do when it creates a freshly formatted file system with flex_bg


Use a journal to ensure filesystem consistency even across unclean shutdowns. Setting the

filesystem feature is equivalent to using the -j option.

Support files larger than 2 terabytes in size.

Filesystem can contain files that are greater than 2GB.

Reserve space so the block group descriptor table may grow in the future. Tune2fs only

supports clearing this filesystem feature.

mmp Enable or disable multiple mount protection (MMP) feature.

quota Enable internal file system quota inodes.

Limit the number of backup superblocks to save space on large filesystems.

Allow the kernel to initialize bitmaps and inode tables lazily, and to keep a high water‐

mark for the unused inodes in a filesystem, to reduce e2fsck(8) time. This first e2fsck

run after enabling this feature will take the full time, but subsequent e2fsck runs will

take only a fraction of the original time, depending on how full the file system is.

After setting or clearing sparse_super, uninit_bg, filetype, or resize_inode filesystem features,

e2fsck(8) must be run on the filesystem to return the filesystem to a consistent state. Tune2fs will

print a message requesting that the system administrator run e2fsck(8) if necessary. After setting

the dir_index feature, e2fsck -D can be run to convert existing directories to the hashed B-tree for‐

mat. Enabling certain filesystem features may prevent the filesystem from being mounted by kernels

which do not support those features. In particular, the uninit_bg and flex_bg features are only sup‐

ported by the ext4 filesystem.

-p mmp_check_interval
Set the desired MMP check interval in seconds. It is 5 seconds by default.

-r reserved-blocks-count
Set the number of reserved filesystem blocks.

-Q quota-options
Sets ‘quota’ feature on the superblock and works on the quota files for the given quota type. Quota

options could be one or more of the following:

Sets/clears user quota inode in the superblock.

Sets/clears group quota inode in the superblock.

-T time-last-checked
Set the time the filesystem was last checked using e2fsck. The time is interpreted using the current

(local) timezone. This can be useful in scripts which use a Logical Volume Manager to make a consis‐

tent snapshot of a filesystem, and then check the filesystem during off hours to make sure it hasn’t

been corrupted due to hardware problems, etc. If the filesystem was clean, then this option can be

used to set the last checked time on the original filesystem. The format of time-last-checked is the

international date format, with an optional time specifier, i.e. YYYYMMDD[HH[MM[SS]]]. The keyword

now is also accepted, in which case the last checked time will be set to the current time.

-u user
Set the user who can use the reserved filesystem blocks. user can be a numerical uid or a user name.

If a user name is given, it is converted to a numerical uid before it is stored in the superblock.

Set the universally unique identifier (UUID) of the filesystem to UUID. The format of the UUID is a

series of hex digits separated by hyphens, like this: “c1b9d5a2-f162-11cf-9ece-0020afc76f16”. The

UUID parameter may also be one of the following:

clear clear the filesystem UUID

random generate a new randomly-generated UUID

time generate a new time-based UUID

The UUID may be used by mount(8), fsck(8), and /etc/fstab(5) (and possibly others) by specifying

UUID=uuid instead of a block special device name like /dev/hda1.

See uuidgen(8) for more information. If the system does not have a good random number generator such

as /dev/random or /dev/urandom, tune2fs will automatically use a time-based UUID instead of a ran‐

domly-generated UUID.


debugfs(8), dumpe2fs(8), e2fsck(8), mke2fs(8), ext4(5)


  • man 8 tune2fs, version tune2fs 1.42.13 (17-May-2015)