mkfs.btrfs(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SIZE UNITS | MULTIPLE DEVICES | FILESYSTEM FEATURES | RUNTIME FEATURES | BLOCK GROUPS, CHUNKS, RAID | PROFILES | DUP PROFILES ON A SINGLE DEVICE | KNOWN ISSUES | AVAILABILITY | SEE ALSO | COLOPHON

MKFS.BTRFS(8)                 Btrfs Manual                 MKFS.BTRFS(8)

NAME         top

       mkfs.btrfs - create a btrfs filesystem

SYNOPSIS         top

       mkfs.btrfs [options] <device> [<device>...]

DESCRIPTION         top

       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.

OPTIONS         top

       -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, unless an SSD
           is detected, in which case it will default to single. The
           detection is based on the value of
           /sys/block/DEV/queue/rotational, where DEV is the short name
           of the device.

           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
           etc). It’s recommended to options --data/--metadata to avoid
           confusion.

           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.

       -U|--uuid <UUID>
           Create the filesystem with the given UUID. The UUID must not
           exist on any filesystem currently present.

       -V|--version
           Print the mkfs.btrfs version and exit.

       --help
           Print help.

SIZE UNITS         top

       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.

MULTIPLE DEVICES         top

       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.

FILESYSTEM FEATURES         top

       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
           (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

RUNTIME FEATURES         top

       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
           (kernel support since 4.5)

           Enable the free space tree (mount option space_cache=v2) for
           persisting the free space cache.

BLOCK GROUPS, CHUNKS, RAID         top

       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

PROFILES         top

       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% │   2/any    │
       ├────────┼────────┼────────┼──────────┼─────────────┼────────────┤
       │        │        │        │          │             │            │
       │RAID1   │   2    │        │          │         50% │   2/any    │
       ├────────┼────────┼────────┼──────────┼─────────────┼────────────┤
       │        │        │        │          │             │            │
       │RAID1C3 │   3    │        │          │         33% │   3/any    │
       ├────────┼────────┼────────┼──────────┼─────────────┼────────────┤
       │        │        │        │          │             │            │
       │RAID1C4 │   4    │        │          │         25% │   4/any    │
       ├────────┼────────┼────────┼──────────┼─────────────┼────────────┤
       │        │        │        │          │             │            │
       │RAID10  │   2    │        │  1 to N  │         50% │   4/any    │
       ├────────┼────────┼────────┼──────────┼─────────────┼────────────┤
       │        │        │        │          │             │            │
       │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.

   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    │
       └─────────┴──────────┴──────────┴──────────┘

DUP PROFILES ON A SINGLE DEVICE         top

       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.

KNOWN ISSUES         top

       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.

AVAILABILITY         top

       mkfs.btrfs is part of btrfs-progs. Please refer to the btrfs wiki
       http://btrfs.wiki.kernel.org for further details.

SEE ALSO         top

       btrfs(5), btrfs(8), btrfs-balance(8), wipefs(8)

COLOPHON         top

       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 2021-04-01.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-24.)  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 man-pages@man7.org

Btrfs v4.6.1                   03/13/2021                  MKFS.BTRFS(8)

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