Provided by: btrfs-progs_5.19-1_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.  The
       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 details 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

       -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 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.

                 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.

                 Mixed profile cannot be used together with other profiles. It can only be set at
                 creation time. Conversion to or from mixed profile is not implemented.

       -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 4KiB.

       -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 on a mounted filesystem.  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.

       --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

       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

          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

       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

              (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

       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


       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

       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).

              (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

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

       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

              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:

           │Profiles │ Redundancy   │ Redundancy │ Redundancy │ Space       │ Min/max        │
           │         │              │            │            │ utilization │ devices        │
           │         │ Copies       │ Parity     │ Striping   │             │                │
           │single   │ 1            │            │            │ 100%        │ 1/any          │
           │DUP      │ 2 / 1 device │            │            │ 50%         │ 1/any     (see │
           │         │              │            │            │             │ note 1)        │
           │RAID0    │ 1            │            │ 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 1 │ device 2 │ device 3 │ device 4 │
                              │A        │ D        │          │          │
                              │B        │          │          │ C        │
                              │C        │          │          │          │
                              │D        │ A        │ B        │          │

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

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

                              │D4       │ B2       │ C4       │ A4       │

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

                              │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.



       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)