Provided by: btrfs-progs_5.16.2-1_amd64 bug


       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.

               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.

           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

           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.

               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.

               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.

           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.

               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 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.

               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

           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.

           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.

           Increase verbosity level, default is 1.

           Print the mkfs.btrfs version and exit.

           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

       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

           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

           (kernel support since 2.6.37)

           mixed data and metadata block groups, also set by option --mixed

           (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

           (kernel support since 3.9)

           extended format for RAID5/6, also enabled if raid5 or raid6 block groups are selected

           (default since btrfs-progs 3.18, kernel support since 3.10)

           reduced-size metadata for extent references, saves a few percent of metadata

           (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

           (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

           (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).

           (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


       The highlevel organizational units of a filesystem are block groups of three types: data,
       metadata and system.

           store data blocks and nothing else

           store internal metadata in b-trees, can store file data if they fit into the inline

           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

           a block group profile type that utilizes RAID-like features on multiple devices:
           striping, mirroring, parity

           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:

       │        │                                  │             │             │
       │ProfileRedundancySpaceMin/max   │
       │        ├──────────────┬────────┬──────────┤ utilizationdevices   │
       │        │              │        │          │             │             │
       │        │    CopiesParityStriping │             │             │
       │        │              │        │          │             │             │
       │single  │      1       │        │          │        100% │    1/any    │
       │        │              │        │          │             │             │
       │DUP     │ 2 / 1 device │        │          │         50% │ 1/any ^(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)     │

           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

       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.

       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.


       │device 1device 2device 3device 4 │
       │         │          │          │          │
       │   A     │    D     │          │          │
       │         │          │          │          │
       │   B     │          │          │    C     │
       │         │          │          │          │
       │   C     │          │          │          │
       │         │          │          │          │
       │   D     │    A     │    B     │          │


       │device 1device 2device 3device 4 │
       │         │          │          │          │
       │   A     │    A     │    D     │          │
       │         │          │          │          │
       │   B     │          │    B     │          │
       │         │          │          │          │
       │   C     │          │    A     │    C     │
       │         │          │          │          │
       │   D     │    D     │    C     │    B     │


       │device 1device 2device 3device 4 │
       │         │          │          │          │
       │   A2    │    C3    │    A3    │    C2    │
       │         │          │          │          │
       │   B1    │    A1    │    D2    │    B3    │
       │         │          │          │          │
       │   C1    │    D3    │    B4    │    D1    │
       │         │          │          │          │
       │   D4    │    B2    │    C4    │    A4    │


       │device 1device 2device 3device 4 │
       │         │          │          │          │
       │   A2    │    C3    │    A3    │    C2    │
       │         │          │          │          │
       │   B1    │    A1    │    D2    │    B3    │
       │         │          │          │          │
       │   C1    │    D3    │    PB    │    D1    │
       │         │          │          │          │
       │   PD    │    B2    │    PC    │    PA    │


       │device 1device 2device 3device 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.



       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 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
             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


       mkfs.btrfs is part of btrfs-progs. Please refer to the btrfs wiki for further details.


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