mirror of
https://github.com/kdave/btrfs-progs
synced 2024-12-25 23:52:17 +00:00
899977cd18
- add raid1c34 - add utilization to the overview table - wording updates Signed-off-by: David Sterba <dsterba@suse.com>
456 lines
17 KiB
Plaintext
456 lines
17 KiB
Plaintext
mkfs.btrfs(8)
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=============
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NAME
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----
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mkfs.btrfs - create a btrfs filesystem
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SYNOPSIS
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--------
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*mkfs.btrfs* [options] <device> [<device>...]
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DESCRIPTION
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-----------
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*mkfs.btrfs* is used to create the btrfs filesystem on a single or multiple
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devices. <device> is typically a block device but can be a file-backed image
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as well. Multiple devices are grouped by UUID of the filesystem.
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Before mounting such filesystem, the kernel module must know all the devices
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either via preceding execution of *btrfs device scan* or using the *device*
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mount option. See section *MULTIPLE DEVICES* for more details.
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OPTIONS
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-------
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*-b|--byte-count <size>*::
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Specify the size of the filesystem. If this option is not used, then
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mkfs.btrfs uses the entire device space for the filesystem.
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*--csum <type>*::
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*--checksum <type>*::
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Specify the checksum algorithm. Default is 'crc32c'. Valid values are 'crc32c',
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'xxhash', 'sha256' or 'blake2'. To mount such filesystem kernel must support the
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checksums as well. See 'CHECKSUM ALGORITHMS' in `btrfs`(5).
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*-d|--data <profile>*::
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Specify the profile for the data block groups. Valid values are 'raid0',
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'raid1', 'raid5', 'raid6', 'raid10' or 'single' or 'dup' (case does not matter).
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+
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See 'DUP PROFILES ON A SINGLE DEVICE' for more details.
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*-m|--metadata <profile>*::
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Specify the profile for the metadata block groups.
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Valid values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or
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'dup' (case does not matter).
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+
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A single device filesystem will default to 'DUP', unless an SSD is detected, in which
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case it will default to 'single'. The detection is based on the value of
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`/sys/block/DEV/queue/rotational`, where 'DEV' is the short name of the device.
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+
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Note that the rotational status can be arbitrarily set by the underlying block
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device driver and may not reflect the true status (network block device, memory-backed
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SCSI devices etc). Use the options '--data/--metadata' to avoid confusion.
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+
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See 'DUP PROFILES ON A SINGLE DEVICE' for more details.
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*-M|--mixed*::
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Normally the data and metadata block groups are isolated. The 'mixed' mode
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will remove the isolation and store both types in the same block group type.
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This helps to utilize the free space regardless of the purpose and is suitable
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for small devices. The separate allocation of block groups leads to a situation
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where the space is reserved for the other block group type, is not available for
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allocation and can lead to ENOSPC state.
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+
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The recommended size for the mixed mode is for filesystems less than 1GiB. The
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soft recommendation is to use it for filesystems smaller than 5GiB. The mixed
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mode may lead to degraded performance on larger filesystems, but is otherwise
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usable, even on multiple devices.
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+
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The 'nodesize' and 'sectorsize' must be equal, and the block group types must
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match.
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+
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NOTE: versions up to 4.2.x forced the mixed mode for devices smaller than 1GiB.
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This has been removed in 4.3+ as it caused some usability issues.
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*-l|--leafsize <size>*::
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Alias for --nodesize. Deprecated.
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*-n|--nodesize <size>*::
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Specify the nodesize, the tree block size in which btrfs stores metadata. The
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default value is 16KiB (16384) or the page size, whichever is bigger. Must be a
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multiple of the sectorsize and a power of 2, but not larger than 64KiB (65536).
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Leafsize always equals nodesize and the options are aliases.
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+
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Smaller node size increases fragmentation but leads to taller b-trees which in
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turn leads to lower locking contention. Higher node sizes give better packing
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and less fragmentation at the cost of more expensive memory operations while
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updating the metadata blocks.
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+
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NOTE: versions up to 3.11 set the nodesize to 4k.
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*-s|--sectorsize <size>*::
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Specify the sectorsize, the minimum data block allocation unit.
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+
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The default value is the page size and is autodetected. If the sectorsize
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differs from the page size, the created filesystem may not be mountable by the
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kernel. Therefore it is not recommended to use this option unless you are going
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to mount it on a system with the appropriate page size.
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*-L|--label <string>*::
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Specify a label for the filesystem. The 'string' should be less than 256
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bytes and must not contain newline characters.
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*-K|--nodiscard*::
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Do not perform whole device TRIM operation on devices that are capable of that.
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This does not affect discard/trim operation when the filesystem is mounted.
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Please see the mount option 'discard' for that in `btrfs`(5).
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*-r|--rootdir <rootdir>*::
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Populate the toplevel subvolume with files from 'rootdir'. This does not
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require root permissions to write the new files or to mount the filesystem.
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NOTE: This option may enlarge the image or file to ensure it's big enough to
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contain the files from 'rootdir'. Since version 4.14.1 the filesystem size is
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not minimized. Please see option '--shrink' if you need that functionality.
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*--shrink*::
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Shrink the filesystem to its minimal size, only works with '--rootdir' option.
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+
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If the destination is a regular file, this option will also truncate the
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file to the minimal size. Otherwise it will reduce the filesystem available
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space. Extra space will not be usable unless the filesystem is mounted and
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resized using 'btrfs filesystem resize'.
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+
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NOTE: prior to version 4.14.1, the shrinking was done automatically.
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*-O|--features <feature1>[,<feature2>...]*::
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A list of filesystem features turned on at mkfs time. Not all features are
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supported by old kernels. To disable a feature, prefix it with '^'.
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+
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See section *FILESYSTEM FEATURES* for more details. To see all available
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features that mkfs.btrfs supports run:
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+
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+mkfs.btrfs -O list-all+
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*-f|--force*::
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Forcibly overwrite the block devices when an existing filesystem is detected.
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By default, mkfs.btrfs will utilize 'libblkid' to check for any known
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filesystem on the devices. Alternatively you can use the `wipefs` utility
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to clear the devices.
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*-q|--quiet*::
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Print only error or warning messages. Options --features or --help are unaffected.
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*-U|--uuid <UUID>*::
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Create the filesystem with the given 'UUID'. The UUID must not exist on any
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filesystem currently present.
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*-V|--version*::
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Print the *mkfs.btrfs* version and exit.
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*--help*::
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Print help.
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*-A|--alloc-start <offset>*::
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*deprecated, will be removed*
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(An option to help debugging chunk allocator.)
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Specify the (physical) offset from the start of the device at which allocations
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start. The default value is zero.
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SIZE UNITS
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----------
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The default unit is 'byte'. All size parameters accept suffixes in the 1024
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base. The recognized suffixes are: 'k', 'm', 'g', 't', 'p', 'e', both uppercase
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and lowercase.
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MULTIPLE DEVICES
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----------------
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Before mounting a multiple device filesystem, the kernel module must know the
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association of the block devices that are attached to the filesystem UUID.
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There is typically no action needed from the user. On a system that utilizes a
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udev-like daemon, any new block device is automatically registered. The rules
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call *btrfs device scan*.
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The same command can be used to trigger the device scanning if the btrfs kernel
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module is reloaded (naturally all previous information about the device
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registration is lost).
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Another possibility is to use the mount options *device* to specify the list of
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devices to scan at the time of mount.
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# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
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NOTE: that this means only scanning, if the devices do not exist in the system,
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mount will fail anyway. This can happen on systems without initramfs/initrd and
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root partition created with RAID1/10/5/6 profiles. The mount action can happen
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before all block devices are discovered. The waiting is usually done on the
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initramfs/initrd systems.
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As of kernel 4.14, RAID5/6 is still considered experimental and shouldn't be
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employed for production use.
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FILESYSTEM FEATURES
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-------------------
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Features that can be enabled during creation time. See also `btrfs`(5) section
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'FILESYSTEM FEATURES'.
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*mixed-bg*::
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(kernel support since 2.6.37)
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+
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mixed data and metadata block groups, also set by option '--mixed'
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*extref*::
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(default since btrfs-progs 3.12, kernel support since 3.7)
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increased hardlink limit per file in a directory to 65536, older kernels
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supported a varying number of hardlinks depending on the sum of all file name
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sizes that can be stored into one metadata block
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*raid56*::
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(kernel support since 3.9)
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extended format for RAID5/6, also enabled if raid5 or raid6 block groups
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are selected
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*skinny-metadata*::
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(default since btrfs-progs 3.18, kernel support since 3.10)
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reduced-size metadata for extent references, saves a few percent of metadata
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*no-holes*::
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(kernel support since 3.14)
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improved representation of file extents where holes are not explicitly
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stored as an extent, saves a few percent of metadata if sparse files are used
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BLOCK GROUPS, CHUNKS, RAID
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--------------------------
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The highlevel organizational units of a filesystem are block groups of three types:
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data, metadata and system.
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*DATA*::
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store data blocks and nothing else
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*METADATA*::
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store internal metadata in b-trees, can store file data if they fit into the
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inline limit
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*SYSTEM*::
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store structures that describe the mapping between the physical devices and the
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linear logical space representing the filesystem
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Other terms commonly used:
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*block group*::
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*chunk*::
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a logical range of space of a given profile, stores data, metadata or both;
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sometimes the terms are used interchangeably
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A typical size of metadata block group is 256MiB (filesystem smaller than
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50GiB) and 1GiB (larger than 50GiB), for data it's 1GiB. The system block group
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size is a few megabytes.
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*RAID*::
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a block group profile type that utilizes RAID-like features on multiple
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devices: striping, mirroring, parity
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*profile*::
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when used in connection with block groups refers to the allocation strategy
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and constraints, see the section 'PROFILES' for more details
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PROFILES
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--------
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There are the following block group types available:
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[ cols="<,^,^,^,>,^",width="60%" ]
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|=============================================================
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.2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Space utilization .2+^.<h| Min/max devices
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^.^h| Copies ^.^h| Parity ^.<h| Striping
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| single | 1 | | | 100% | 1/any
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| DUP | 2 / 1 device | | | 50% | 1/any ^(see note 1)^
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| RAID0 | | | 1 to N | 100% | 2/any
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| RAID1 | 2 | | | 50% | 2/any
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| RAID1C3 | 3 | | | 33% | 3/any
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| RAID1C4 | 4 | | | 25% | 4/any
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| RAID10 | 2 | | 1 to N | 50% | 4/any
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| RAID5 | 1 | 1 | 2 to N-1 | (N-1)/N | 2/any ^(see note 2)^
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| RAID6 | 1 | 2 | 3 to N-2 | (N-2)/N | 3/any ^(see note 3)^
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|=============================================================
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WARNING: It's not recommended to create filesystems with RAID0/1/10/5/6
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profiles on partitions from the same device. Neither redundancy nor
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performance will be improved.
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'Note 1:' DUP may exist on more than 1 device if it starts on a single device and
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another one is added. Since version 4.5.1, *mkfs.btrfs* will let you create DUP
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on multiple devices without restrictions.
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'Note 2:' It's not recommended to use 2 devices with RAID5. In that case,
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parity stripe will contain the same data as the data stripe, making RAID5
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degraded to RAID1 with more overhead.
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'Note 3:' It's also not recommended to use 3 devices with RAID6, unless you
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want to get effectively 3 copies in a RAID1-like manner (but not exactly that).
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'Note 4:' Since kernel 5.5 it's possible to use RAID1C3 as replacement for
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RAID6, higher space cost but reliable.
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PROFILE LAYOUT
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~~~~~~~~~~~~~~
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For the following examples, assume devices numbered by 1, 2, 3 and 4, data or
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metadata blocks A, B, C, D, with possible stripes eg. A1, A2 that would be
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logically A, etc. For parity profiles PA and QA are parity and syndrom,
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associated with the given stripe. The simple layouts single or DUP are left
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out. Actual physical block placement on devices depends on current state of
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the free/allocated space and may appear random. All devices are assumed to be
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present at the time of the blocks would have been written.
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RAID1
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[ cols="^,^,^,^",width="50%", options="header" ]
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|===
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| device 1 | device 2 | device 3 | device 4
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| A | D | |
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| B | | | C
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| C | | |
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| D | A | B |
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|===
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RAID1C3
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[ cols="^,^,^,^",width="50%", options="header" ]
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|===
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| device 1 | device 2 | device 3 | device 4
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| A | A | D |
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| B | | B |
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| C | | A | C
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| D | D | C | B
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|===
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RAID0
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[ cols="^,^,^,^",width="50%", options="header" ]
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|===
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| device 1 | device 2 | device 3 | device 4
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| A2 | C3 | A3 | C2
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| B1 | A1 | D2 | B3
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| C1 | D3 | B4 | D1
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| D4 | B2 | C4 | A4
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|===
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RAID5
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[ cols="^,^,^,^",width="50%", options="header" ]
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|===
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| device 1 | device 2 | device 3 | device 4
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| A2 | C3 | A3 | C2
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| B1 | A1 | D2 | B3
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| C1 | D3 | PB | D1
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| PD | B2 | PC | PA
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|===
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RAID6
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[ cols="^,^,^,^",width="50%", options="header" ]
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|===
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| device 1 | device 2 | device 3 | device 4
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| A2 | QC | QA | C2
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| B1 | A1 | D2 | QB
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| C1 | QD | PB | D1
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| PD | B2 | PC | PA
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|===
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DUP PROFILES ON A SINGLE DEVICE
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-------------------------------
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The mkfs utility will let the user create a filesystem with profiles that write
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the logical blocks to 2 physical locations. Whether there are really 2
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physical copies highly depends on the underlying device type.
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For example, a SSD drive can remap the blocks internally to a single copy--thus
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deduplicating them. This negates the purpose of increased redundancy and just
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wastes filesystem space without providing the expected level of redundancy.
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The duplicated data/metadata may still be useful to statistically improve the
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chances on a device that might perform some internal optimizations. The actual
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details are not usually disclosed by vendors. For example we could expect that
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not all blocks get deduplicated. This will provide a non-zero probability of
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recovery compared to a zero chance if the single profile is used. The user
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should make the tradeoff decision. The deduplication in SSDs is thought to be
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widely available so the reason behind the mkfs default is to not give a false
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sense of redundancy.
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As another example, the widely used USB flash or SD cards use a translation
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layer between the logical and physical view of the device. The data lifetime
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may be affected by frequent plugging. The memory cells could get damaged,
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hopefully not destroying both copies of particular data in case of DUP.
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The wear levelling techniques can also lead to reduced redundancy, even if the
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device does not do any deduplication. The controllers may put data written in
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a short timespan into the same physical storage unit (cell, block etc). In case
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this unit dies, both copies are lost. BTRFS does not add any artificial delay
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between metadata writes.
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The traditional rotational hard drives usually fail at the sector level.
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In any case, a device that starts to misbehave and repairs from the DUP copy
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should be replaced! *DUP is not backup*.
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KNOWN ISSUES
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------------
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**SMALL FILESYSTEMS AND LARGE NODESIZE**
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The combination of small filesystem size and large nodesize is not recommended
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in general and can lead to various ENOSPC-related issues during mount time or runtime.
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Since mixed block group creation is optional, we allow small
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filesystem instances with differing values for 'sectorsize' and 'nodesize'
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to be created and could end up in the following situation:
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# mkfs.btrfs -f -n 65536 /dev/loop0
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btrfs-progs v3.19-rc2-405-g976307c
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See http://btrfs.wiki.kernel.org for more information.
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Performing full device TRIM (512.00MiB) ...
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Label: (null)
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UUID: 49fab72e-0c8b-466b-a3ca-d1bfe56475f0
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Node size: 65536
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Sector size: 4096
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Filesystem size: 512.00MiB
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Block group profiles:
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Data: single 8.00MiB
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Metadata: DUP 40.00MiB
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System: DUP 12.00MiB
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SSD detected: no
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Incompat features: extref, skinny-metadata
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Number of devices: 1
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Devices:
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ID SIZE PATH
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1 512.00MiB /dev/loop0
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# mount /dev/loop0 /mnt/
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mount: mount /dev/loop0 on /mnt failed: No space left on device
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The ENOSPC occurs during the creation of the UUID tree. This is caused
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by large metadata blocks and space reservation strategy that allocates more
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than can fit into the filesystem.
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AVAILABILITY
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------------
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*mkfs.btrfs* is part of btrfs-progs.
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Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for
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further details.
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SEE ALSO
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--------
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`btrfs`(5),
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`btrfs`(8),
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`wipefs`(8)
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