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

NAME
       mkfs.btrfs - create a btrfs filesystem

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

DESCRIPTION
       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.

OPTIONS
       -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.

       -d|--data <profile>
           Specify the profile for the data block groups. Valid values are raid0,
           raid1, raid5, raid6, raid10 or single or dup (case does not matter).

           See DUP PROFILES ON A SINGLE DEVICE for more.

       -m|--metadata <profile>
           Specify the profile for the metadata block groups. Valid values are raid0,
           raid1, raid5, raid6, raid10, single or dup, (case does not matter).

           A single device filesystem will default to DUP, unless a SSD is detected.
           Then 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). Use the options --data/--metadata
           to avoid confusion.

           See DUP PROFILES ON A SINGLE DEVICE for more details.

       -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 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 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

       -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.

       -A|--alloc-start <offset>
           deprecated, will be removed (An option to help debugging chunk allocator.)
           Specify the (physical) offset from the start of the device at which
           allocations start. The default value is zero.

SIZE UNITS
       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
       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.

       As of kernel 4.14, RAID5/6 is still considered experimental and shouldn’t be
       employed for production use.

FILESYSTEM FEATURES
       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

BLOCK GROUPS, CHUNKS, RAID
       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
       There are the following block group types available:

       ┌────────┬────────────────────────────────────┬─────────────┐
       │        │                                    │             │
       │Profile │ Redundancy                         │   Min/max   │
       │        ├──────────────┬────────┬────────────┤   devices   │
       │        │              │        │            │             │
       │        │    Copies    │ Parity │  Striping  │             │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │single  │      1       │        │            │    1/any    │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │  DUP   │ 2 / 1 device │        │            │ 1/any ^(see │
       │        │              │        │            │ note 1)     │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │ RAID0  │              │        │   1 to N   │    2/any    │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │ RAID1  │      2       │        │            │    2/any    │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │RAID10  │      2       │        │   1 to N   │    4/any    │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │ RAID5  │      1       │   1    │ 2 to N - 1 │ 2/any ^(see │
       │        │              │        │            │ note 2)     │
       ├────────┼──────────────┼────────┼────────────┼─────────────┤
       │        │              │        │            │             │
       │ RAID6  │      1       │   2    │ 3 to N - 2 │ 3/any ^(see │
       │        │              │        │            │ note 3)     │
       └────────┴──────────────┴────────┴────────────┴─────────────┘

           Warning
           It’s not recommended to build btrfs 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.

       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).
       N-copies RAID1 is not implemented.

DUP PROFILES ON A SINGLE DEVICE
       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
       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
       mkfs.btrfs is part of btrfs-progs. Please refer to the btrfs wiki
       http://btrfs.wiki.kernel.org for further details.

SEE ALSO
       btrfs(5), btrfs(8), wipefs(8)

Btrfs v4.20.1                          01/23/2019                         MKFS.BTRFS(8)

Usage: mkfs.btrfs [options] dev [ dev ... ]
Options:
  allocation profiles:
        -d|--data PROFILE       data profile, raid0, raid1, raid5, raid6, raid10, dup or single
        -m|--metadata PROFILE   metadata profile, values like for data profile
        -M|--mixed              mix metadata and data together
  features:
        -n|--nodesize SIZE      size of btree nodes
        -s|--sectorsize SIZE    data block size (may not be mountable by current kernel)
        -O|--features LIST      comma separated list of filesystem features (use '-O list-all' to list features)
        -L|--label LABEL        set the filesystem label
        -U|--uuid UUID          specify the filesystem UUID (must be unique)
  creation:
        -b|--byte-count SIZE    set filesystem size to SIZE (on the first device)
        -r|--rootdir DIR        copy files from DIR to the image root directory
        --shrink                (with --rootdir) shrink the filled filesystem to minimal size
        -K|--nodiscard          do not perform whole device TRIM
        -f|--force              force overwrite of existing filesystem
  general:
        -q|--quiet              no messages except errors
        -V|--version            print the mkfs.btrfs version and exit
        --help                  print this help and exit
  deprecated:
        -A|--alloc-start START  the offset to start the filesystem
        -l|--leafsize SIZE      deprecated, alias for nodesize