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

OPTIONS         top

       -A|--alloc-start <offset>
           (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.

       -b|--byte-count <size>
           Specify the size of the filesystem. If this option is not used,
           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.

           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.

       -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, but
           not larger than 64KiB (65536). Leafsize always equals nodesize
           and the options are aliases.

           Smaller node size increases fragmentation but lead to higher
           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

               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.

           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 and does not mount the

       -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

           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.

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

           Print the mkfs.btrfs version and exit.

           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.


       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

           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 4.9, RAID5/6 is still considered experimental and
       shouldn’t be employed for production use.


       Features that can be enabled during creation time. See also btrfs(5)

           (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

           (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

           (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


       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 limit

           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.

           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

PROFILES         top

       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)    │

           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.


       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

       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 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

       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


       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 filesystem.

AVAILABILITY         top

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

SEE ALSO         top

       btrfs(5), btrfs(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 
       If you have a bug report for this manual page, see 
       This page was obtained from the project's upstream Git repository 
       on 2017-05-03.  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

Btrfs v4.6.1                     02/18/2017                    MKFS.BTRFS(8)

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