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


   -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. The 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, 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


   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

       (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


   There are the following block group types available:

   Profile  Redundancy                           Min/max   
               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 prfiles 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

   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.



   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(5), btrfs(8), wipefs(8)

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