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MKFS.BTRFS(8) BTRFS MKFS.BTRFS(8) 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 de‐ vice 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 pre‐ ceding 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 possi‐ bly 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 de‐ vice 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 gen‐ eral. 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/rota‐ tional) 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 de‐ vice driver and may not reflect the true status (network block device, mem‐ ory-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 uti‐ lize 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 multi‐ ple 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 meta‐ data 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 de‐ fault, mkfs.btrfs will utilize libblkid to check for any known filesystem on the de‐ vices. 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 du‐ plicate 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 back‐ ward compatibility. SIZE UNITS The default unit is byte. All size parameters accept suffixes in the 1024 base. The recog‐ nized 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 re‐ loaded (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 any‐ way. 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 discov‐ ered. 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 FEA‐ TURES. 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 perfor‐ mance 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 de‐ vices 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 back‐ ward 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 con‐ straints, see the section PROFILES for more details PROFILES There are the following block group types available: ─────────────────────────────────────────────────────────────────────────────────────── Profiles Redundancy Redundancy Redundancy Space utiliza‐ Min/max de‐ tion vices 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) ─────────────────────────────────────────────────────────────────────────────────────── │ │ │ │ │ │ │ WA│RNING: │ │ │ │ │ │ │It's not r│ecommended to c│reate filesys│tems with RAI│D0/1/10/5/6 profi│les on partitions│from │the same d│evice. Neither│redundancy n│or performanc│e will be improve│d. │ │ │ │ │ │ │ │ No│te 1: DUP m│ay exist on mor│e than 1 devi│ce if it star│ts on a single de│vice and another│ one is│added. Sin│ce version 4.5.│1, mkfs.btrfs│will let you│create DUP on mu│ltiple devices wi│thout re│strictions.│ │ │ │ │ │ │ │ │ │ │ │ │ No│te 2: It'│s not recommen│ded to use 2 │devices with │RAID5. In that ca│se, parity stripe│will co│ntain the s│ame data as the│data stripe,│making RAID5│degraded to RAID│1 with more overh│ead. │ │ │ │ │ │ │ No│te 3: It's │also not recomm│ended to use │3 devices wit│h RAID6, unless y│ou want to get e│ffec‐ ti│vely 3 copi│es in a RAID1-l│ike manner (b│ut not exactl│y that). │ │ │ │ │ │ │ │ │ No│te 4: Sinc│e kernel 5.5 it│'s possible t│o use RAID1C3│as replacement f│or RAID6, higher │space co│st but reli│able. │ │ │ │ │ │ │ │ │ │ │ │ No│te 5: Since│kernel 5.15 it│'s possible t│o use (mount,│convert profiles│) RAID0 on one d│evice an│d RAID10 on│two devices. │ │ │ │ │ │ │ │ │ │ │ │ PROFIL│E LAYOUT │ │ │ │ │ │ Fo│r the follo│wing examples, │assume device│s numbered by│1, 2, 3 and 4, d│ata or metadata b│locks A,│B, C, D, w│ith possible st│ripes e.g. A1│, A2 that wou│ld be logically A│, etc. For parity│pro‐ fi│les PA an│d QA are parity│and syndrome│, associated │with the given st│ripe. The simple│lay‐ ou│ts single o│r DUP are left │out. Actual │physical bloc│k placement on de│vices depends on │cur‐ re│nt state │of the free/all│ocated space │and may appea│r random. All dev│ices are assumed │to be pr│esent at th│e time of the b│locks would h│ave been writ│ten. │ │ │ │ │ │ │ │ │ RAID1 │ │ │ │ │ │ │ │ │ ┌────┼─────┬──────┼───┬────────┼─┬──────────┐ │ │ │ │ │ dev│ice 1 │ devic│e 2 │ device │3 │ device 4 │ │ │ │ │ ├────┼─────┼──────┼───┼────────┼─┼──────────┤ │ │ │ │ │ A │ │ D │ │ │ │ │ │ │ │ │ ├────┼─────┼──────┼───┼────────┼─┼──────────┤ │ │ │ │ │ B │ │ │ │ │ │ C │ │ │ │ │ ├────┼─────┼──────┼───┼────────┼─┼──────────┤ │ │ │ │ │ C │ │ │ │ │ │ │ │ │ │ │ ├────┼─────┼──────┼───┼────────┼─┼──────────┤ │ │ │ │ │ D │ │ A │ │ B │ │ │ │ │ │ │ └────┼─────┴──────┼───┴────────┼─┴──────────┘ │ │ │ │ │ │ │ │ │ RAID1C│3 │ │ │ │ │ │ ┌──────────┬──────────┬──────────┬──────────┐ │ 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 DEVIC│E │ │ │ │ The mkfs utility will l│et the user│create a f│ilesystem w│ith profile│s that write the logical blocks to 2 physical l│ocations. W│hether ther│e are reall│y 2 physica│l copies highly depends on the underlying device t│ype. │ │ │ │ │ │ │ │ │ For example, a SSD driv│e can remap│the blocks│internally│to a singl│e copy--thus deduplicating them. This negates the │purpose of │increased r│edundancy a│nd just was│tes filesystem space with‐ out providing the expec│ted level o│f redundanc│y. │ │ │ │ │ │ │ The duplicated data/met│adata may s│till be use│ful to stat│istically i│mprove the chances on a device that might pe│rform some│internal o│ptimization│s. The actu│al details are not usually disclosed by vendors. F│or example │we could ex│pect that n│ot all bloc│ks get deduplicated. This will provide a non-zer│o probabili│ty of recov│ery compare│d to a zero│chance if the single pro‐ file is used. The user │should make│the trade│off decisi│on. The d│eduplication in SSDs is thought to be widely │available s│o the reaso│n behind th│e mkfs defa│ult is to not give a false sense of redundancy. │ │ │ │ │ │ │ │ │ │ As another example, the│widely use│d USB flash│or SD card│s use a tra│nslation layer between the logical and physical vi│ew of the d│evice. The │data lifeti│me may be a│ffected by frequent plug‐ ging. The memory cells│could get │damaged, ho│pefully not│destroying│both copies of particular data in case of DUP. │ │ │ │ │ │ │ │ │ │ The wear levelling tech│niques can │also lead t│o reduced r│edundancy, │even if the device does not do any deduplicat│ion. The co│ntrollers m│ay put data│written in│a short timespan into the same physical storage u│nit (cell, │block etc).│In case th│is unit die│s, both copies are lost. BTRFS does not add any │artificial │delay betwe│en metadata│writes. │ │ │ │ │ │ The traditional rotatio│nal hard dr│ives usuall│y fail at t│he sector l│evel. │ │ │ │ │ In any case, a devic│e that star│ts to misbe│have and re│pairs from │the DUP copy should be re‐ placed! DUP is not back│up. │ │ │ │ │ │ │ │ │ KNOWN ISSUES │ │ │ │ │ SMALL FILESYSTEMS AND L│ARGE NODESI│ZE │ │ │ │ │ │ │ │ The combination of smal│l filesyste│m size and │large nodes│ize is not │recommended in general and can lead to various ENO│SPC-related│issues dur│ing mount t│ime or runt│ime. │ │ │ │ │ Since mixed block group│creation i│s optional,│we allow s│mall filesy│stem instances with dif‐ fering values for se│ctorsize a│nd nodesize│to be crea│ted and cou│ld end up in the following situation: │ │ │ │ │ │ │ │ │ │ # mkfs.btrfs -f -n 6│5536 /dev/l│oop0 │ │ │ btrfs-progs v3.19-rc│2-405-g9763│07c │ │ │ See https://btrfs.re│adthedocs.i│o for more │information│. │ │ │ │ │ │ Performing full devi│ce TRIM (51│2.00MiB) ..│. │ │ 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) 6.6.3 Mar 31, 2024 MKFS.BTRFS(8)
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