Provided by: btrfs-tools_4.4-1_i386 bug


       mkfs.btrfs - create a btrfs filesystem


       mkfs.btrfs [-A|--alloc-start <alloc-start>] [-b|--byte-count
       <byte-count>] [-d|--data <data-profile>] [-m|--metadata <metadata
       profile>] [-M|--mixed] [-l|--leafsize <leafsize>] [-n|--nodesize
       <nodesize>] [-s|--sectorsize <sectorsize>] [-L|--label <label>]
       [-K|--nodiscard] [-r|--rootdir <rootdir>] [-O|--features
       <feature1>[,<feature2>...]] [-U|--uuid <UUID>] [-f|--force]
       [-q|--quiet] [--help] [-V|--version] <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

       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.


       -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

           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

           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. Thie mixed mode may lead to degraded performance
           on larger filesystems, but is otherwise usable, even on multiple

           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.

       -r|--rootdir <rootdir>
           Populate the toplevel subvolume with files from rootdir. This does
           not require root permissions and does not mount the filesystem.

       -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.


       The default unit is byte. All size parameters accept suffixes in the
       1024 base. The recognized suffixes are: k, m, g, t, 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


           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

           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

           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 interchangably

           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


       There are the following block group types available:

       │        │                                    │          │
       │ProfileRedundancyMin/max  │
       │        ├──────────────┬────────┬────────────┤ devices  │
       │        │              │        │            │          │
       │        │    CopiesParityStriping  │          │
       │        │              │        │            │          │
       │single  │      1       │        │            │  1/any   │
       │        │              │        │            │          │
       │  DUP   │ 2 / 1 device │        │            │ 1/1 (see │
       │        │              │        │            │ note)    │
       │        │              │        │            │          │
       │ 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   │
       │        │              │        │            │          │
       │ RAID6  │      1       │   2    │ 3 to N - 2 │  3/any   │

       Note: DUP may exist on more than 1 device if it starts on a single
       device and another one is added, but mkfs.btrfs will not let you create
       DUP on multiple devices.


       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
       redunancy and just wastes space.

       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.
       As another example, the widely used USB flash or SD cards use a
       translation layer. The data lifetime may be affected by frequent
       plugging. The memory cells could get damaged, hopefully not destroying
       both copies of particular data.

       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.



       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.


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


       btrfs(8), wipefs(8)