Provided by: btrfs-progs_6.6.3-1.1build2_amd64 
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. The
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.
The default block group profiles for data and metadata depend on number of devices and
possibly other factors. It's recommended to use specific profiles but the defaults should
be OK and allowing future conversions to other profiles. Please see options -d and -m for
further details and btrfs-balance(8) for the profile conversion post mkfs.
OPTIONS
-b|--byte-count <size>
Specify the size of each device as seen by the filesystem. If not set, the entire
device size is used. The total filesystem size will be sum of all device sizes, for
a single device filesystem the option effectively specifies the size of the
filesystem.
--csum <type>, --checksum <type>
Specify the checksum algorithm. Default is crc32c. Valid values are crc32c, xxhash,
sha256 or blake2. To mount such filesystem kernel must support the checksums as
well. See section CHECKSUM ALGORITHMS in btrfs(5).
-d|--data <profile>
Specify the profile for the data block groups. Valid values are raid0, raid1,
raid1c3, raid1c4, raid5, raid6, raid10 or single or dup (case does not matter).
See section DUP PROFILES ON A SINGLE DEVICE for more details.
On multiple devices, the default was raid0 until version 5.7, while it is single
since version 5.8. You can still select raid0 manually, but it was not suitable as
default.
-m|--metadata <profile>
Specify the profile for the metadata block groups. Valid values are raid0, raid1,
raid1c3, raid1c4, raid5, raid6, raid10, single or dup (case does not matter).
Default on a single device filesystem is DUP and is recommended for metadata in
general. The duplication might not be necessary in some use cases and it's up to
the user to changed that at mkfs time or later. This depends on hardware that could
potentially deduplicate the blocks again but this cannot be detected at mkfs time.
NOTE:
Up to version 5.14 there was a detection of a SSD device (more precisely if it's
a rotational device, determined by the contents of file
/sys/block/DEV/queue/rotational) that used to select single. This has changed in
version 5.15 to be always dup.
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, real block device behind some additional device
mapper layer, etc). It's recommended to always set the options --data/--metadata
to avoid confusion and unexpected results.
See section DUP PROFILES ON A SINGLE DEVICE for more details.
On multiple devices the default is raid1.
-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.
Mixed profile cannot be used together with other profiles. It can only be set at
creation time. Conversion to or from mixed profile is not implemented.
-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 4KiB.
-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 running
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 block device 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.
Resets any previous effects of --verbose.
-U|--uuid <UUID>
Create the filesystem with the given UUID. For a single-device filesystem, you can
duplicate the UUID. However, for a multi-device filesystem, the UUID must not
already exist on any currently present filesystem.
--device-uuid <UUID>
Create the filesystem with the given device-uuid UUID (also known as UUID_SUB in
blkid). For a single device filesystem, you can duplicate the device-uuid.
However, used for a multi-device filesystem this option will not work at the
moment.
-v|--verbose
Increase verbosity level, default is 1.
-V|--version
Print the mkfs.btrfs version and exit.
--help Print help.
-l|--leafsize <size>
Removed in 6.0, used to be alias for --nodesize.
-R|--runtime-features <feature1>[,<feature2>...]
Removed in 6.3, was used to specify features not affecting on-disk format. Now all
such features are merged into -O|--features option. The option -R will stay for
backward compatibility.
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:
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.
WARNING:
RAID5/6 has known problems and should not be used in production.
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
(default since btrfs-progs 5.15, 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
zoned (kernel support since 5.12)
zoned mode, data allocation and write friendly to zoned/SMR/ZBC/ZNS devices, see
ZONED MODE in btrfs(5), the mode is automatically selected when a zoned device is
detected
quota (kernel support since 3.4)
Enable quota support (qgroups). The qgroup accounting will be consistent, can be
used together with --rootdir. See also btrfs-quota(8).
free-space-tree
(default since btrfs-progs 5.15, kernel support since 4.5)
Enable the free space tree (mount option space_cache=v2) for persisting the free
space cache in a b-tree. This is built on top of the COW mechanism and has better
performance than v1.
Offline conversion from filesystems that don't have this feature enabled at mkfs
time is possible, see btrfstune(8).
Online conversion can be done by mounting with space_cache=v2, this is sufficient
to be done one time.
block-group-tree
(kernel support since 6.1)
Enable a dedicated b-tree for block group items, this greatly reduces mount time
for large filesystems due to better data locality that avoids seeking. On
rotational devices the large size is considered starting from the 2-4TiB. Can be
used on other types of devices (SSD, NVMe, ...) as well.
Offline conversion from filesystems that don't have this feature enabled at mkfs
time is possible, see btrfstune(8). Online conversion is not possible.
raid-stripe-tree
(kernel support since 6.7)
New tree for logical file extent mapping where the physical mapping may not match
on multiple devices. this is now used in zoned mode to implement RAID0/RAID1*
profiles, but can be used in non-zoned mode as well. The support for RAID56 is in
development and will eventually fix the problems with the current implementation.
This is a backward incompatible feature and has to be enabled at mkfs time.
squota (kernel support since 6.7)
Enable simple quota accounting (squotas). This is an alternative to qgroups with a
smaller performance impact but no notion of shared vs. exclusive usage.
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:
┌─────────┬──────────────┬────────────┬────────────┬─────────────┬────────────────┐
│Profiles │ Redundancy │ Redundancy │ Redundancy │ Space │ Min/max │
│ │ │ │ │ utilization │ devices │
│ │ Copies │ Parity │ Striping │ │ │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│single │ 1 │ │ │ 100% │ 1/any │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│DUP │ 2 / 1 device │ │ │ 50% │ 1/any (see │
│ │ │ │ │ │ note 1) │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID0 │ 1 │ │ 1 to N │ 100% │ 1/any (see │
│ │ │ │ │ │ note 5) │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID1 │ 2 │ │ │ 50% │ 2/any │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID1C3 │ 3 │ │ │ 33% │ 3/any │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID1C4 │ 4 │ │ │ 25% │ 4/any │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID10 │ 2 │ │ 1 to N │ 50% │ 2/any (see │
│ │ │ │ │ │ note 5) │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID5 │ 1 │ 1 │ 2 to N-1 │ (N-1)/N │ 2/any (see │
│ │ │ │ │ │ note 2) │
├─────────┼──────────────┼────────────┼────────────┼─────────────┼────────────────┤
│RAID6 │ 1 │ 2 │ 3 to N-2 │ (N-2)/N │ 3/any (see │
│ │ │ │ │ │ note 3) │
└─────────┴──────────────┴────────────┴────────────┴─────────────┴────────────────┘
WARNING:
It's not recommended to create filesystems 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
without restrictions.
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).
Note 4: Since kernel 5.5 it's possible to use RAID1C3 as replacement for RAID6, higher
space cost but reliable.
Note 5: Since kernel 5.15 it's possible to use (mount, convert profiles) RAID0 on one
device and RAID10 on two devices.
PROFILE LAYOUT
For the following examples, assume devices numbered by 1, 2, 3 and 4, data or metadata
blocks A, B, C, D, with possible stripes e.g. A1, A2 that would be logically A, etc. For
parity profiles PA and QA are parity and syndrome, associated with the given stripe. The
simple layouts single or DUP are left out. Actual physical block placement on devices
depends on current state of the free/allocated space and may appear random. All devices
are assumed to be present at the time of the blocks would have been written.
RAID1
┌─────────┬──────────┬──────────┬──────────┐
│device 1 │ device 2 │ device 3 │ device 4 │
├─────────┼──────────┼──────────┼──────────┤
│A │ D │ │ │
├─────────┼──────────┼──────────┼──────────┤
│B │ │ │ C │
└─────────┴──────────┴──────────┴──────────┘
│C │ │ │ │
├─────────┼──────────┼──────────┼──────────┤
│D │ A │ B │ │
└─────────┴──────────┴──────────┴──────────┘
RAID1C3
┌─────────┬──────────┬──────────┬──────────┐
│device 1 │ device 2 │ device 3 │ device 4 │
├─────────┼──────────┼──────────┼──────────┤
│A │ A │ D │ │
├─────────┼──────────┼──────────┼──────────┤
│B │ │ B │ │
├─────────┼──────────┼──────────┼──────────┤
│C │ │ A │ C │
├─────────┼──────────┼──────────┼──────────┤
│D │ D │ C │ B │
└─────────┴──────────┴──────────┴──────────┘
RAID0
┌─────────┬──────────┬──────────┬──────────┐
│device 1 │ device 2 │ device 3 │ device 4 │
├─────────┼──────────┼──────────┼──────────┤
│A2 │ C3 │ A3 │ C2 │
├─────────┼──────────┼──────────┼──────────┤
│B1 │ A1 │ D2 │ B3 │
├─────────┼──────────┼──────────┼──────────┤
│C1 │ D3 │ B4 │ D1 │
├─────────┼──────────┼──────────┼──────────┤
│D4 │ B2 │ C4 │ A4 │
└─────────┴──────────┴──────────┴──────────┘
RAID5
┌─────────┬──────────┬──────────┬──────────┐
│device 1 │ device 2 │ device 3 │ device 4 │
├─────────┼──────────┼──────────┼──────────┤
│A2 │ C3 │ A3 │ C2 │
├─────────┼──────────┼──────────┼──────────┤
│B1 │ A1 │ D2 │ B3 │
├─────────┼──────────┼──────────┼──────────┤
│C1 │ D3 │ PB │ D1 │
├─────────┼──────────┼──────────┼──────────┤
│PD │ B2 │ PC │ PA │
└─────────┴──────────┴──────────┴──────────┘
RAID6
┌─────────┬──────────┬──────────┬──────────┐
│device 1 │ device 2 │ device 3 │ device 4 │
├─────────┼──────────┼──────────┼──────────┤
│A2 │ QC │ QA │ C2 │
├─────────┼──────────┼──────────┼──────────┤
│B1 │ A1 │ D2 │ QB │
├─────────┼──────────┼──────────┼──────────┤
│C1 │ QD │ PB │ D1 │
├─────────┼──────────┼──────────┼──────────┤
│PD │ B2 │ PC │ PA │
└─────────┴──────────┴──────────┴──────────┘
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 https://btrfs.readthedocs.io 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. Please refer to the documentation at
https://btrfs.readthedocs.io.
SEE ALSO
btrfs(5), btrfs(8), btrfs-balance(8), wipefs(8)