SYNOPSIS
--------
-*mkfs.btrfs*
-$$[-A|--alloc-start <alloc-start>]$$
-$$[-b|--byte-count <byte-count>]$$
-$$[-d|--data <data-profile>]$$
-$$[-m|--metadata <metadata profile>]$$
-$$[-M|--mixed]$$
-$$[-l|--leafsize <leafsize>]$$
-$$[-n|--nodesize <nodesize>]$$
-$$[-s|--sectorsize <sectorsize>]$$
-$$[-L|--label <label>]$$
-$$[-K|--nodiscard]$$
-$$[-r|--rootdir <rootdir>]$$
-$$[-O|--features <feature1>[,<feature2>...]]$$
-$$[-U|--uuid <UUID>]$$
-$$[-f|--force]$$
-$$[-q|--quiet]$$
-$$[--help]$$
-$$[-V|--version]$$
-$$<device> [<device>...]$$
+*mkfs.btrfs* [options] <device> [<device>...]
DESCRIPTION
-----------
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,
+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', (case does not matter).
+'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.
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.
-This is because SSDs can remap the blocks internally to a single copy thus
-deduplicating them which negates the purpose of increased metadata redunancy
-and just wastes space.
+
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
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
+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.
+
*-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.
+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 lead to higher b-trees which in
+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.
*-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 and does not mount the filesystem.
+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
*--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', 'e', both uppercase and
-lowercase.
+base. The recognized suffixes are: 'k', 'm', 'g', 't', 'p', 'e', both uppercase
+and lowercase.
MULTIPLE DEVICES
----------------
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*::
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
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 group*::
*chunk*::
a logical range of space of a given profile, stores data, metadata or both;
-sometimes the terms are used interchangably
+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
There are the following block group types available:
-[ width="60%",options="header" ]
+[ cols="^,^,^,^,^",width="60%" ]
|=============================================================
-| Profile | Redundancy | Striping | Min/max devices
-| single | 1 copy | n/a | 1/any
-| DUP | 2 copies / 1 device | n/a | 1/1
-| RAID0 | n/a | 1 to N | 2/any
-| RAID10 | 2 copies | 1 to N | 4/any
-| RAID5 | 2 copies | 3 to N - 1 | 2/any
-| RAID6 | 3 copies | 3 to N - 2 | 3/any
+.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
------------
SEE ALSO
--------
-`btrfs`(8), `wipefs`(8)
+`btrfs`(5),
+`btrfs`(8),
+`wipefs`(8)
projects / platform / upstream / btrfs-progs.git / blobdiff