Provided by: btrfs-progs_4.16.1-2ubuntu1_amd64 bug

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:

       ┌────────┬────────────────────────────────────┬─────────────────┐
       │        │                                    │                 │
       │ProfileRedundancyMin/max devices │
       │        ├──────────────┬────────┬────────────┤                 │
       │        │              │        │            │                 │
       │        │    CopiesParityStriping  │                 │
       ├────────┼──────────────┼────────┼────────────┼─────────────────┤
       │        │              │        │            │                 │
       │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)