SYNOPSIS
--------
-*mkfs.btrfs*
-$$[-A|--alloc-start <alloc-start>]$$
-$$[-b|--byte-count <byte-count>]$$
-$$[-d|--data <data-profile>]$$
-$$[-f|--force]$$
-$$[-n|--nodesize <nodesize>]$$
-$$[-l|--leafsize <leafsize>]$$
-$$[-L|--label <label>]$$
-$$[-m|--metadata <metadata profile>]$$
-$$[-M|--mixed]$$
-$$[-q|--quiet]$$
-$$[-s|--sectorsize <sectorsize>]$$
-$$[-r|--rootdir <rootdir>]$$
-$$[-K|--nodiscard]$$
-$$[-O|--features <feature1>[,<feature2>...]]$$
-$$[-U|--uuid <UUID>]$$
-$$[-h]$$
-$$[-V|--version]$$
-$$<device> [<device>...]$$
+*mkfs.btrfs* [options] <device> [<device>...]
DESCRIPTION
-----------
-*mkfs.btrfs*
-is used to create a btrfs filesystem (usually in a disk partition, or an array
-of disk partitions).
+*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.
-<device>
-is the special file corresponding to the device (e.g /dev/sdXX ).
-If multiple devices are specified, btrfs is created
-spanning across the specified devices.
+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>::
-Specify the offset from the start of the device at which to start allocations
-in this btrfs filesystem. The default value is zero, or the start of the
-device. This option is intended only for debugging filesystem resize
-operations.
-
--b|--byte-count <size>::
-Specify the size of the resultant filesystem. If this option is not used,
-mkfs.btrfs uses all the available storage for the filesystem.
-
--d|--data <type>::
-Specify how the data must be spanned across the devices specified. Valid
-values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10' or 'single'.
-
--f|--force::
-Force overwrite when an existing filesystem is detected on the device.
-By default, mkfs.btrfs will not write to the device if it suspects that
-there is a filesystem or partition table on the device already.
-
--l|--leafsize <size>::
-Alias for --nodesize. Deprecated.
+*-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.
--n|--nodesize <size>::
-Specify the nodesize, the tree block size in which btrfs stores
-data. The default value is 16KB (16384) or the page size, whichever is
-bigger. Must be a multiple of the sectorsize, but not larger than 65536.
-Leafsize always equals nodesize and the options are aliases.
+*-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.
--L|--label <name>::
-Specify a label for the filesystem.
+*-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.
+
-NOTE: <name> should be less than 256 characters.
+See 'DUP PROFILES ON A SINGLE DEVICE' for more details.
--m|--metadata <profile>::
-Specify how metadata must be spanned across the devices specified. Valid
-values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or 'dup'.
+*-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.
+
-Single device
-will have dup set by default except in the case of SSDs which will default to
-single. This is because SSDs can remap blocks internally so duplicate blocks
-could end up in the same erase block which negates the benefits of doing
-metadata duplication.
+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.
--M|--mixed::
-Mix data and metadata chunks together for more efficient space
-utilization. This feature incurs a performance penalty in
-larger filesystems. It is recommended for use with filesystems
-of 1 GiB or smaller.
+*-l|--leafsize <size>*::
+Alias for --nodesize. Deprecated.
--q|--quiet::
-Print only error or warning messages. Options --features or --help are unaffected.
+*-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>::
+*-s|--sectorsize <size>*::
Specify the sectorsize, the minimum data block allocation unit.
+
-The default
-value is the page size. If the sectorsize differs from the page size, the
-created filesystem may not be mountable by current 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.
+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.
--r|--rootdir <rootdir>::
-Specify a directory to copy into the newly created btrfs filesystem.
+*-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: '-r' option is done completely in userland, and don't need root
-privilege 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.
--K|--nodiscard::
-Do not perform whole device TRIM operation by default.
+*--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>...]::
+*-O|--features <feature1>[,<feature2>...]*::
A list of filesystem features turned on at mkfs time. Not all features are
-supported by old kernels.
+supported by old kernels. To disable a feature, prefix it with '^'.
+
-To see all features run:
+See section *FILESYSTEM FEATURES* for more details. To see all available
+features that mkfs.btrfs supports run:
+
+mkfs.btrfs -O list-all+
--U|--uuid <UUID>::
-Create the filesystem with the specified UUID, which must not already exist on
-the system.
+*-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.
--V|--version::
+*-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.
--h::
+*--help*::
Print help.
-UNIT
-----
-As default the unit is the byte, however it is possible to append a suffix
-to the arguments like 'k' for KBytes, 'm' for MBytes...
+*-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:
+
+[ 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/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
------------
-*btrfs* is part of btrfs-progs.
+*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)
+`btrfs`(5),
+`btrfs`(8),
+`wipefs`(8)
projects / platform / upstream / btrfs-progs.git / blobdiff