Documentation/mkfs.btrfs.asciidoc - pub/scm/linux/kernel/git/mason/btrfs-progs - Git at Google

 mkfs.btrfs(8)
 =============

 NAME
 ----
 mkfs.btrfs - create a btrfs filesystem

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

 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
 -------
 *-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.

 *-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. Thie 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, 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 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.

 *-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+

 *-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.

 SIZE UNITS
 ----------
 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.

 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.

 FILESYSTEM FEATURES
 -------------------

 *mixed-bg*::
 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*::
 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*::
 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 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.

 *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:

 [ cols="^,^,^,^,^",width="60%" ]
 |=============================================================
 .2+^.<h| Profile   3+^.^h| Redundancy           .2+^.<h| Min/max devices
       ^.^h| Copies   ^.^h| Parity     ^.<h| Striping
 | 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.

 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 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 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`(8), `wipefs`(8)
	mkfs.btrfs(8)
	=============

	NAME
	----
	mkfs.btrfs - create a btrfs filesystem

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

	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
	-------
	-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.

	-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. Thie 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, 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 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.

	-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+

	-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.

	SIZE UNITS
	----------
	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.

	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.

	FILESYSTEM FEATURES
	-------------------

	mixed-bg::
	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::
	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::
	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 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.

	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:

	[ cols="^,^,^,^,^",width="60%" ]
	\|=============================================================
	.2+^.<h\| Profile 3+^.^h\| Redundancy .2+^.<h\| Min/max devices
	^.^h\| Copies ^.^h\| Parity ^.<h\| Striping
	\| 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.

	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 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 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`(8), `wipefs`(8)