-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 the 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:

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:

-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:

-s|--sectorsize <size>

Specify the sectorsize, the minimum data block allocation unit.

NOTE:

By default, the value is 4KiB, but it can be manually set to match the system page size (e.g. using command getconf PAGE_SIZE). However, if the sector size is different from the page size, the resulting filesystem may not be mountable by the current kernel, apart from the default 4KiB. Hence, using this option is not advised unless you intend to mount it on a system with the suitable page size.

In experimental build the size 2K (2048) is also allowed and needs kernel that also allows that value, then it works in the subpage mode.

-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.

Directories can be created as subvolumes, see also option --subvol. Hardlinks are detected and created in the filesystem image.

NOTE:

--compress <algo>[:<level>]

Try to compress files when using --rootdir. Supported values for algo are no (the default), zstd, lzo or zlib. The optional value level is a compression level, 1..15 for zstd, 1..9 for zlib.

It is recommended to use the highest level to achieve maximum space savings. Compression at mkfs time is not as constrained as in kernel where it's desirable to use less CPU load. Otherwise the default level is 3.

As with the kernel, mkfs.btrfs won't write compressed extents when they would be larger than the uncompressed versions, and will set file attribute NOCOMPRESS if its beginning is found to be incompressible.

NOTE:

-u|--subvol <type>:<subdir>

Specify that subdir is to be created as a subvolume rather than a regular directory. The option --rootdir must also be specified, and subdir must be an existing subdirectory within it. This option can be specified multiple times.

The type is an optional additional modifier. Valid choices are:

Only one of default and default-ro may be specified.

If you wish to create a subvolume with a name containing a colon and you don't want this to be parsed as containing a modifier, you can prefix the path with ./:

$ mkfs.btrfs --rootdir dir --subvol ./ro:subdir /dev/loop0

If there are hardlinks inside rootdir and subdir will split the subvolumes, like the following case:

rootdir/
|- hardlink1
|- hardlink2
|- subdir/  <- will be a subvolume
   |- hardlink3

In that case we cannot create hardlink3 as hardlinks of hardlink1 and hardlink2 because hardlink3 will be inside a new subvolume.

--inode-flags <flags>:<path>

Specify that path to have inode flags, other than the default one (which implies data COW and data checksum). The option --rootdir must also be specified. This option can be specified multiple times.

The supported flag(s) are:

flags can be separated by comma (,).

Children inodes will inherit the flags from their parent inodes, like the following case:

rootdir/
|- file1
|- file2
|- dir/
   |- file3

In that case, if --inode-flags nodatacow:dir is specified, both dir and file3 will have the nodatacow flag.

And this option also works with --subvol option, but the inode flag of each subvolume is independent and will not inherit from the parent directory. (The same as the kernel behavior.)

NOTE:

--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:

-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, builtin features and exit.

--help

Print help.

-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.

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, CONFIG_BTRFS_DEBUG/CONFIG_BTRFS_EXPERIMENTAL)

Separate 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.

NOTE:

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.

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

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