|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
MKFS.BTRFS(8) Btrfs Manual MKFS.BTRFS(8)
mkfs.btrfs - create a btrfs filesystem
mkfs.btrfs [options] <device> [<device>...]
mkfs.btrfs is used to create the btrfs filesystem on a single or
multiple devices. <device> is typically a block device but can be
a file-backed image as well. Multiple devices are grouped by UUID
of the filesystem.
Before mounting such filesystem, the kernel module must know all
the devices either via preceding execution of btrfs device scan or
using the device mount option. See section MULTIPLE DEVICES for
more details.
The default block group profiles for data and metadata depend on
number of devices and possibly other factors. It’s recommended to
use specific profiles but the defaults should be OK and allowing
future conversions to other profiles. Please see options -d and -m
for further detals and btrfs-balance(8) for the profile conversion
post mkfs.
-b|--byte-count <size>
Specify the size of the filesystem. If this option is not
used, then mkfs.btrfs uses the entire device space for the
filesystem.
--csum <type>, --checksum <type>
Specify the checksum algorithm. Default is crc32c. Valid
values are crc32c, xxhash, sha256 or blake2. To mount such
filesystem kernel must support the checksums as well. See
CHECKSUM ALGORITHMS in btrfs(5).
-d|--data <profile>
Specify the profile for the data block groups. Valid values
are raid0, raid1, raid1c3, raid1c4, raid5, raid6, raid10 or
single or dup (case does not matter).
See DUP PROFILES ON A SINGLE DEVICE for more details.
On multiple devices, the default was raid0 until version 5.7,
while it is single since version 5.8. You can still select
raid0 manually, but it was not suitable as default.
-m|--metadata <profile>
Specify the profile for the metadata block groups. Valid
values are raid0, raid1, raid1c3, raid1c4, raid5, raid6,
raid10, single or dup (case does not matter).
Default on a single device filesystem is DUP and is
recommended for metadata in general. The duplication might not
be necessary in some use cases and it’s up to the user to
changed that at mkfs time or later. This depends on hardware
that could potentially deduplicate the blocks again but this
cannot be detected at mkfs time.
NOTE
Up to version 5.14 there was a detection of a SSD device
(more precisely if it’s a rotational device, determined by
the contents of file /sys/block/DEV/queue/rotational) that
used to select single. This has changed in version 5.15 to
be always dup.
Note that the rotational status can be arbitrarily set by
the underlying block device driver and may not reflect the
true status (network block device, memory-backed SCSI
devices, real block device behind some additional device
mapper layer, etc). It’s recommended to always set the
options --data/--metadata to avoid confusion and
unexpected results.
See DUP PROFILES ON A SINGLE DEVICE for more details.
On multiple devices the default is raid1.
-M|--mixed
Normally the data and metadata block groups are isolated. The
mixed mode will remove the isolation and store both types in
the same block group type. This helps to utilize the free
space regardless of the purpose and is suitable for small
devices. The separate allocation of block groups leads to a
situation where the space is reserved for the other block
group type, is not available for allocation and can lead to
ENOSPC state.
The recommended size for the mixed mode is for filesystems
less than 1GiB. The soft recommendation is to use it for
filesystems smaller than 5GiB. The mixed mode may lead to
degraded performance on larger filesystems, but is otherwise
usable, even on multiple devices.
The nodesize and sectorsize must be equal, and the block group
types must match.
Note
versions up to 4.2.x forced the mixed mode for devices
smaller than 1GiB. This has been removed in 4.3+ as it
caused some usability issues.
-l|--leafsize <size>
Alias for --nodesize. Deprecated.
-n|--nodesize <size>
Specify the nodesize, the tree block size in which btrfs
stores metadata. The default value is 16KiB (16384) or the
page size, whichever is bigger. Must be a multiple of the
sectorsize and a power of 2, but not larger than 64KiB
(65536). Leafsize always equals nodesize and the options are
aliases.
Smaller node size increases fragmentation but leads to taller
b-trees which in turn leads to lower locking contention.
Higher node sizes give better packing and less fragmentation
at the cost of more expensive memory operations while updating
the metadata blocks.
Note
versions up to 3.11 set the nodesize to 4k.
-s|--sectorsize <size>
Specify the sectorsize, the minimum data block allocation
unit.
The default value is the page size and is autodetected. If the
sectorsize differs from the page size, the created filesystem
may not be mountable by the running kernel. Therefore it is
not recommended to use this option unless you are going to
mount it on a system with the appropriate page size.
-L|--label <string>
Specify a label for the filesystem. The string should be less
than 256 bytes and must not contain newline characters.
-K|--nodiscard
Do not perform whole device TRIM operation on devices that are
capable of that. This does not affect discard/trim operation
when the filesystem is mounted. Please see the mount option
discard for that in btrfs(5).
-r|--rootdir <rootdir>
Populate the toplevel subvolume with files from rootdir. This
does not require root permissions to write the new files or to
mount the filesystem.
Note
This option may enlarge the image or file to ensure it’s
big enough to contain the files from rootdir. Since
version 4.14.1 the filesystem size is not minimized.
Please see option --shrink if you need that functionality.
--shrink
Shrink the filesystem to its minimal size, only works with
--rootdir option.
If the destination block device is a regular file, this option
will also truncate the file to the minimal size. Otherwise it
will reduce the filesystem available space. Extra space will
not be usable unless the filesystem is mounted and resized
using btrfs filesystem resize.
Note
prior to version 4.14.1, the shrinking was done
automatically.
-O|--features <feature1>[,<feature2>...]
A list of filesystem features turned on at mkfs time. Not all
features are supported by old kernels. To disable a feature,
prefix it with ^.
See section FILESYSTEM FEATURES for more details. To see all
available features that mkfs.btrfs supports run:
mkfs.btrfs -O list-all
-R|--runtime-features <feature1>[,<feature2>...]
A list of features that be can enabled at mkfs time, otherwise
would have to be turned on a mounted filesystem. Although no
runtime feature is enabled by default, to disable a feature,
prefix it with ^.
See section RUNTIME FEATURES for more details. To see all
available runtime features that mkfs.btrfs supports run:
mkfs.btrfs -R list-all
-f|--force
Forcibly overwrite the block devices when an existing
filesystem is detected. By default, mkfs.btrfs will utilize
libblkid to check for any known filesystem on the devices.
Alternatively you can use the wipefs utility to clear the
devices.
-q|--quiet
Print only error or warning messages. Options --features or
--help are unaffected. Resets any previous effects of
--verbose.
-U|--uuid <UUID>
Create the filesystem with the given UUID. The UUID must not
exist on any filesystem currently present.
-v|--verbose
Increase verbosity level, default is 1.
-V|--version
Print the mkfs.btrfs version and exit.
--help
Print help.
The default unit is byte. All size parameters accept suffixes in
the 1024 base. The recognized suffixes are: k, m, g, t, p, e, both
uppercase and lowercase.
Before mounting a multiple device filesystem, the kernel module
must know the association of the block devices that are attached
to the filesystem UUID.
There is typically no action needed from the user. On a system
that utilizes a udev-like daemon, any new block device is
automatically registered. The rules call btrfs device scan.
The same command can be used to trigger the device scanning if the
btrfs kernel module is reloaded (naturally all previous
information about the device registration is lost).
Another possibility is to use the mount options device to specify
the list of devices to scan at the time of mount.
# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
Note
that this means only scanning, if the devices do not exist in
the system, mount will fail anyway. This can happen on systems
without initramfs/initrd and root partition created with
RAID1/10/5/6 profiles. The mount action can happen before all
block devices are discovered. The waiting is usually done on
the initramfs/initrd systems.
RAID5/6 has known problems and should not be used in production.
Features that can be enabled during creation time. See also
btrfs(5) section FILESYSTEM FEATURES.
mixed-bg
(kernel support since 2.6.37)
mixed data and metadata block groups, also set by option
--mixed
extref
(default since btrfs-progs 3.12, kernel support since 3.7)
increased hardlink limit per file in a directory to 65536,
older kernels supported a varying number of hardlinks
depending on the sum of all file name sizes that can be stored
into one metadata block
raid56
(kernel support since 3.9)
extended format for RAID5/6, also enabled if raid5 or raid6
block groups are selected
skinny-metadata
(default since btrfs-progs 3.18, kernel support since 3.10)
reduced-size metadata for extent references, saves a few
percent of metadata
no-holes
(default since btrfs-progs 5.15, kernel support since 3.14)
improved representation of file extents where holes are not
explicitly stored as an extent, saves a few percent of
metadata if sparse files are used
zoned
(kernel support since 5.12)
zoned mode, data allocation and write friendly to
zoned/SMR/ZBC/ZNS devices, see ZONED MODE in btrfs(5), the
mode is automatically selected when a zoned device is detected
Features that are typically enabled on a mounted filesystem, eg.
by a mount option or by an ioctl. Some of them can be enabled
early, at mkfs time. This applies to features that need to be
enabled once and then the status is permanent, this does not
replace mount options.
quota
(kernel support since 3.4)
Enable quota support (qgroups). The qgroup accounting will be
consistent, can be used together with --rootdir. See also
btrfs-quota(8).
free-space-tree
(default since btrfs-progs 5.15, kernel support since 4.5)
Enable the free space tree (mount option space_cache=v2) for
persisting the free space cache.
The highlevel organizational units of a filesystem are block
groups of three types: data, metadata and system.
DATA
store data blocks and nothing else
METADATA
store internal metadata in b-trees, can store file data if
they fit into the inline limit
SYSTEM
store structures that describe the mapping between the
physical devices and the linear logical space representing the
filesystem
Other terms commonly used:
block group, chunk
a logical range of space of a given profile, stores data,
metadata or both; sometimes the terms are used interchangeably
A typical size of metadata block group is 256MiB (filesystem
smaller than 50GiB) and 1GiB (larger than 50GiB), for data
it’s 1GiB. The system block group size is a few megabytes.
RAID
a block group profile type that utilizes RAID-like features on
multiple devices: striping, mirroring, parity
profile
when used in connection with block groups refers to the
allocation strategy and constraints, see the section PROFILES
for more details
There are the following block group types available:
┌─────────┬────────────────────────────┬─────────────┬────────────┐
│ │ │ │ │
│ Profile │ Redundancy │ Space │ Min/max │
│ ├────────┬────────┬──────────┤ utilization │ devices │
│ │ │ │ │ │ │
│ │ Copies │ Parity │ Striping │ │ │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ single │ 1 │ │ │ 100% │ 1/any │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ DUP │ 2 / 1 │ │ │ 50% │ 1/any │
│ │ device │ │ │ │ ^(see note │
│ │ │ │ │ │ 1) │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID0 │ │ │ 1 to N │ 100% │ 1/any │
│ │ │ │ │ │ ^(see note │
│ │ │ │ │ │ 5) │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID1 │ 2 │ │ │ 50% │ 2/any │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID1C3 │ 3 │ │ │ 33% │ 3/any │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID1C4 │ 4 │ │ │ 25% │ 4/any │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID10 │ 2 │ │ 1 to N │ 50% │ 2/any │
│ │ │ │ │ │ ^(see note │
│ │ │ │ │ │ 5) │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID5 │ 1 │ 1 │ 2 to N-1 │ (N-1)/N │ 2/any │
│ │ │ │ │ │ ^(see note │
│ │ │ │ │ │ 2) │
├─────────┼────────┼────────┼──────────┼─────────────┼────────────┤
│ │ │ │ │ │ │
│ RAID6 │ 1 │ 2 │ 3 to N-2 │ (N-2)/N │ 3/any │
│ │ │ │ │ │ ^(see note │
│ │ │ │ │ │ 3) │
└─────────┴────────┴────────┴──────────┴─────────────┴────────────┘
Warning
It’s not recommended to create filesystems with RAID0/1/10/5/6
profiles on partitions from the same device. Neither
redundancy nor performance will be improved.
Note 1: DUP may exist on more than 1 device if it starts on a
single device and another one is added. Since version 4.5.1,
mkfs.btrfs will let you create DUP on multiple devices without
restrictions.
Note 2: It’s not recommended to use 2 devices with RAID5. In that
case, parity stripe will contain the same data as the data stripe,
making RAID5 degraded to RAID1 with more overhead.
Note 3: It’s also not recommended to use 3 devices with RAID6,
unless you want to get effectively 3 copies in a RAID1-like manner
(but not exactly that).
Note 4: Since kernel 5.5 it’s possible to use RAID1C3 as
replacement for RAID6, higher space cost but reliable.
Note 5: Since kernel 5.15 it’s possible to use (mount, convert
profiles) RAID0 on one device and RAID10 on two devices.
PROFILE LAYOUT
For the following examples, assume devices numbered by 1, 2, 3 and
4, data or metadata blocks A, B, C, D, with possible stripes eg.
A1, A2 that would be logically A, etc. For parity profiles PA and
QA are parity and syndrom, associated with the given stripe. The
simple layouts single or DUP are left out. Actual physical block
placement on devices depends on current state of the
free/allocated space and may appear random. All devices are
assumed to be present at the time of the blocks would have been
written.
RAID1
┌──────────┬──────────┬──────────┬──────────┐
│ device 1 │ device 2 │ device 3 │ device 4 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ A │ D │ │ │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ B │ │ │ C │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ C │ │ │ │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ D │ A │ B │ │
└──────────┴──────────┴──────────┴──────────┘
RAID1C3
┌──────────┬──────────┬──────────┬──────────┐
│ device 1 │ device 2 │ device 3 │ device 4 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ A │ A │ D │ │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ B │ │ B │ │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ C │ │ A │ C │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ D │ D │ C │ B │
└──────────┴──────────┴──────────┴──────────┘
RAID0
┌──────────┬──────────┬──────────┬──────────┐
│ device 1 │ device 2 │ device 3 │ device 4 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ A2 │ C3 │ A3 │ C2 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ B1 │ A1 │ D2 │ B3 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ C1 │ D3 │ B4 │ D1 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ D4 │ B2 │ C4 │ A4 │
└──────────┴──────────┴──────────┴──────────┘
RAID5
┌──────────┬──────────┬──────────┬──────────┐
│ device 1 │ device 2 │ device 3 │ device 4 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ A2 │ C3 │ A3 │ C2 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ B1 │ A1 │ D2 │ B3 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ C1 │ D3 │ PB │ D1 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ PD │ B2 │ PC │ PA │
└──────────┴──────────┴──────────┴──────────┘
RAID6
┌──────────┬──────────┬──────────┬──────────┐
│ device 1 │ device 2 │ device 3 │ device 4 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ A2 │ QC │ QA │ C2 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ B1 │ A1 │ D2 │ QB │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ C1 │ QD │ PB │ D1 │
├──────────┼──────────┼──────────┼──────────┤
│ │ │ │ │
│ PD │ B2 │ PC │ PA │
└──────────┴──────────┴──────────┴──────────┘
The mkfs utility will let the user create a filesystem with
profiles that write the logical blocks to 2 physical locations.
Whether there are really 2 physical copies highly depends on the
underlying device type.
For example, a SSD drive can remap the blocks internally to a
single copy—thus deduplicating them. This negates the purpose of
increased redundancy and just wastes filesystem space without
providing the expected level of redundancy.
The duplicated data/metadata may still be useful to statistically
improve the chances on a device that might perform some internal
optimizations. The actual details are not usually disclosed by
vendors. For example we could expect that not all blocks get
deduplicated. This will provide a non-zero probability of recovery
compared to a zero chance if the single profile is used. The user
should make the tradeoff decision. The deduplication in SSDs is
thought to be widely available so the reason behind the mkfs
default is to not give a false sense of redundancy.
As another example, the widely used USB flash or SD cards use a
translation layer between the logical and physical view of the
device. The data lifetime may be affected by frequent plugging.
The memory cells could get damaged, hopefully not destroying both
copies of particular data in case of DUP.
The wear levelling techniques can also lead to reduced redundancy,
even if the device does not do any deduplication. The controllers
may put data written in a short timespan into the same physical
storage unit (cell, block etc). In case this unit dies, both
copies are lost. BTRFS does not add any artificial delay between
metadata writes.
The traditional rotational hard drives usually fail at the sector
level.
In any case, a device that starts to misbehave and repairs from
the DUP copy should be replaced! DUP is not backup.
SMALL FILESYSTEMS AND LARGE NODESIZE
The combination of small filesystem size and large nodesize is not
recommended in general and can lead to various ENOSPC-related
issues during mount time or runtime.
Since mixed block group creation is optional, we allow small
filesystem instances with differing values for sectorsize and
nodesize to be created and could end up in the following
situation:
# mkfs.btrfs -f -n 65536 /dev/loop0
btrfs-progs v3.19-rc2-405-g976307c
See http://btrfs.wiki.kernel.org for more information.
Performing full device TRIM (512.00MiB) ...
Label: (null)
UUID: 49fab72e-0c8b-466b-a3ca-d1bfe56475f0
Node size: 65536
Sector size: 4096
Filesystem size: 512.00MiB
Block group profiles:
Data: single 8.00MiB
Metadata: DUP 40.00MiB
System: DUP 12.00MiB
SSD detected: no
Incompat features: extref, skinny-metadata
Number of devices: 1
Devices:
ID SIZE PATH
1 512.00MiB /dev/loop0
# mount /dev/loop0 /mnt/
mount: mount /dev/loop0 on /mnt failed: No space left on device
The ENOSPC occurs during the creation of the UUID tree. This is
caused by large metadata blocks and space reservation strategy
that allocates more than can fit into the filesystem.
mkfs.btrfs is part of btrfs-progs. Please refer to the btrfs wiki
http://btrfs.wiki.kernel.org for further details.
btrfs(5), btrfs(8), btrfs-balance(8), wipefs(8)
This page is part of the btrfs-progs (btrfs filesystem tools)
project. Information about the project can be found at
⟨https://btrfs.wiki.kernel.org/index.php/Btrfs_source_repositories⟩.
If you have a bug report for this manual page, see
⟨https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#How_do_I_report_bugs_and_issues.3F⟩.
This page was obtained from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git⟩
on 2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-06-23.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to [email protected]
Btrfs v5.16.1 02/06/2022 MKFS.BTRFS(8)
Pages that refer to this page: repart.d(5), btrfs(8), btrfs-balance(8), btrfs-check(8), btrfs-convert(8), btrfs-device(8), btrfs-filesystem(8), btrfs-find-root(8), btrfs-image(8), btrfs-inspect-internal(8), btrfs-map-logical(8), btrfs-property(8), btrfs-qgroup(8), btrfs-quota(8), btrfs-receive(8), btrfs-replace(8), btrfs-rescue(8), btrfs-restore(8), btrfs-scrub(8), btrfs-send(8), btrfs-subvolume(8), btrfstune(8), [email protected](8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|