Manpages - btrfs.5

btrfs-man5 - topics about the BTRFS filesystem (mount options, supported file attributes and other)

This document describes topics related to BTRFS that are not specific to the tools. Currently covers:

mount options

filesystem features

checksum algorithms

compression

filesystem exclusive operations

filesystem limits

bootloader support

file attributes

zoned mode

control device

filesystems with multiple block group profiles

seeding device

raid56 status and recommended practices

storage model

hardware considerations

This section describes mount options specific to BTRFS. For the generic mount options please refer to *mount*(8) manpage. The options are sorted alphabetically (discarding the no prefix).

Note

most mount options apply to the whole filesystem and only options in the first mounted subvolume will take effect. This is due to lack of implementation and may change in the future. This means that (for example) you can’t set per-subvolume nodatacow, nodatasum, or compress using mount options. This should eventually be fixed, but it has proved to be difficult to implement correctly within the Linux VFS framework.

Mount options are processed in order, only the last occurrence of an option takes effect and may disable other options due to constraints (see eg. nodatacow and compress). The output of mount command shows which options have been applied.

acl, noacl

(default: on)

Enable/disable support for Posix Access Control Lists (ACLs). See the *acl*(5) manual page for more information about ACLs.

The support for ACL is build-time configurable (BTRFS_FS_POSIX_ACL) and mount fails if acl is requested but the feature is not compiled in.

autodefrag, noautodefrag

(since: 3.0, default: off)

Enable automatic file defragmentation. When enabled, small random writes into files (in a range of tens of kilobytes, currently it’s 64K) are detected and queued up for the defragmentation process. Not well suited for large database workloads.

The read latency may increase due to reading the adjacent blocks that make up the range for defragmentation, successive write will merge the blocks in the new location.

#+begin_quote

Warning

Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as well as with Linux stable kernel versions ≥ 3.10.31, ≥ 3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data (for example files copied with cp –reflink, snapshots or de-duplicated data). This may cause considerable increase of space usage depending on the broken up reflinks.

#+end_quote

barrier, nobarrier

(default: on)

Ensure that all IO write operations make it through the device cache and are stored permanently when the filesystem is at its consistency checkpoint. This typically means that a flush command is sent to the device that will synchronize all pending data and ordinary metadata blocks, then writes the superblock and issues another flush.

The write flushes incur a slight hit and also prevent the IO block scheduler to reorder requests in a more effective way. Disabling barriers gets rid of that penalty but will most certainly lead to a corrupted filesystem in case of a crash or power loss. The ordinary metadata blocks could be yet unwritten at the time the new superblock is stored permanently, expecting that the block pointers to metadata were stored permanently before.

On a device with a volatile battery-backed write-back cache, the nobarrier option will not lead to filesystem corruption as the pending blocks are supposed to make it to the permanent storage.

check_int, check_int_data, *check_int_print_mask=*/value/

(since: 3.0, default: off)

These debugging options control the behavior of the integrity checking module (the BTRFS_FS_CHECK_INTEGRITY config option required). The main goal is to verify that all blocks from a given transaction period are properly linked.

check_int enables the integrity checker module, which examines all block write requests to ensure on-disk consistency, at a large memory and CPU cost.

check_int_data includes extent data in the integrity checks, and implies the check_int option.

check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values as defined in fs/btrfs/check-integrity.c, to control the integrity checker module behavior.

See comments at the top of fs/btrfs/check-integrity.c for more information.

clear_cache

Force clearing and rebuilding of the disk space cache if something has gone wrong. See also: space_cache.

*commit=*/seconds/

(since: 3.12, default: 30)

Set the interval of periodic transaction commit when data are synchronized to permanent storage. Higher interval values lead to larger amount of unwritten data, which has obvious consequences when the system crashes. The upper bound is not forced, but a warning is printed if it’s more than 300 seconds (5 minutes). Use with care.

compress, compress=*/type[:level]/, *compress-force, *compress-force=*/type[:level]/

(default: off, level support since: 5.1)

Control BTRFS file data compression. Type may be specified as zlib, lzo, zstd or no (for no compression, used for remounting). If no type is specified, zlib is used. If compress-force is specified, then compression will always be attempted, but the data may end up uncompressed if the compression would make them larger.

Both zlib and zstd (since version 5.1) expose the compression level as a tunable knob with higher levels trading speed and memory (zstd) for higher compression ratios. This can be set by appending a colon and the desired level. Zlib accepts the range [1, 9] and zstd accepts [1, 15]. If no level is set, both currently use a default level of 3. The value 0 is an alias for the default level.

Otherwise some simple heuristics are applied to detect an incompressible file. If the first blocks written to a file are not compressible, the whole file is permanently marked to skip compression. As this is too simple, the compress-force is a workaround that will compress most of the files at the cost of some wasted CPU cycles on failed attempts. Since kernel 4.15, a set of heuristic algorithms have been improved by using frequency sampling, repeated pattern detection and Shannon entropy calculation to avoid that.

#+begin_quote

Note

If compression is enabled, nodatacow and nodatasum are disabled.

#+end_quote

datacow, nodatacow

(default: on)

Enable data copy-on-write for newly created files. Nodatacow implies nodatasum, and disables compression. All files created under nodatacow are also set the NOCOW file attribute (see *chattr*(1)).

#+begin_quote

Note

If nodatacow or nodatasum are enabled, compression is disabled.

Updates in-place improve performance for workloads that do frequent overwrites, at the cost of potential partial writes, in case the write is interrupted (system crash, device failure). #+end_quote

datasum, nodatasum

(default: on)

Enable data checksumming for newly created files. Datasum implies datacow, ie. the normal mode of operation. All files created under nodatasum inherit the “no checksums” property, however there’s no corresponding file attribute (see *chattr*(1)).

#+begin_quote

Note

If nodatacow or nodatasum are enabled, compression is disabled.

There is a slight performance gain when checksums are turned off, the corresponding metadata blocks holding the checksums do not need to updated. The cost of checksumming of the blocks in memory is much lower than the IO, modern CPUs feature hardware support of the checksumming algorithm. #+end_quote

degraded

(default: off)

Allow mounts with less devices than the RAID profile constraints require. A read-write mount (or remount) may fail when there are too many devices missing, for example if a stripe member is completely missing from RAID0.

Since 4.14, the constraint checks have been improved and are verified on the chunk level, not an the device level. This allows degraded mounts of filesystems with mixed RAID profiles for data and metadata, even if the device number constraints would not be satisfied for some of the profiles.

Example: metadata — raid1, data — single, devices — /dev/sda, /dev/sdb

Suppose the data are completely stored on sda, then missing sdb will not prevent the mount, even if 1 missing device would normally prevent (any) single profile to mount. In case some of the data chunks are stored on sdb, then the constraint of single/data is not satisfied and the filesystem cannot be mounted.

*device=*/devicepath/

Specify a path to a device that will be scanned for BTRFS filesystem during mount. This is usually done automatically by a device manager (like udev) or using the btrfs device scan command (eg. run from the initial ramdisk). In cases where this is not possible the device mount option can help.

#+begin_quote

Note

booting eg. a RAID1 system may fail even if all filesystem’s device paths are provided as the actual device nodes may not be discovered by the system at that point.

#+end_quote

discard, discard=sync, discard=async, nodiscard

(default: off, async support since: 5.6)

Enable discarding of freed file blocks. This is useful for SSD devices, thinly provisioned LUNs, or virtual machine images; however, every storage layer must support discard for it to work.

In the synchronous mode (sync or without option value), lack of asynchronous queued TRIM on the backing device TRIM can severely degrade performance, because a synchronous TRIM operation will be attempted instead. Queued TRIM requires newer than SATA revision 3.1 chipsets and devices.

The asynchronous mode (async) gathers extents in larger chunks before sending them to the devices for TRIM. The overhead and performance impact should be negligible compared to the previous mode and it’s supposed to be the preferred mode if needed.

If it is not necessary to immediately discard freed blocks, then the fstrim tool can be used to discard all free blocks in a batch. Scheduling a TRIM during a period of low system activity will prevent latent interference with the performance of other operations. Also, a device may ignore the TRIM command if the range is too small, so running a batch discard has a greater probability of actually discarding the blocks.

enospc_debug, noenospc_debug

(default: off)

Enable verbose output for some ENOSPC conditions. It’s safe to use but can be noisy if the system reaches near-full state.

*fatal_errors=*/action/

(since: 3.4, default: bug)

Action to take when encountering a fatal error.

bug

#+begin_quote BUG() on a fatal error, the system will stay in the crashed state and may be still partially usable, but reboot is required for full operation

panic

panic() on a fatal error, depending on other system configuration, this may be followed by a reboot. Please refer to the documentation of kernel boot parameters, eg. panic, oops or crashkernel.

#+end_quote

flushoncommit, noflushoncommit

(default: off)

This option forces any data dirtied by a write in a prior transaction to commit as part of the current commit, effectively a full filesystem sync.

This makes the committed state a fully consistent view of the file system from the application’s perspective (i.e. it includes all completed file system operations). This was previously the behavior only when a snapshot was created.

When off, the filesystem is consistent but buffered writes may last more than one transaction commit.

*fragment=*/type/

(depends on compile-time option BTRFS_DEBUG, since: 4.4, default: off)

A debugging helper to intentionally fragment given type of block groups. The type can be data, metadata or all. This mount option should not be used outside of debugging environments and is not recognized if the kernel config option BTRFS_DEBUG is not enabled.

nologreplay

(default: off, even read-only)

The tree-log contains pending updates to the filesystem until the full commit. The log is replayed on next mount, this can be disabled by this option. See also treelog. Note that nologreplay is the same as norecovery.

#+begin_quote

Warning

currently, the tree log is replayed even with a read-only mount! To disable that behaviour, mount also with nologreplay.

#+end_quote

*max_inline=*/bytes/

(default: min(2048, page size) )

Specify the maximum amount of space, that can be inlined in a metadata B-tree leaf. The value is specified in bytes, optionally with a K suffix (case insensitive). In practice, this value is limited by the filesystem block size (named sectorsize at mkfs time), and memory page size of the system. In case of sectorsize limit, there’s some space unavailable due to leaf headers. For example, a 4k sectorsize, maximum size of inline data is about 3900 bytes.

Inlining can be completely turned off by specifying 0. This will increase data block slack if file sizes are much smaller than block size but will reduce metadata consumption in return.

#+begin_quote

Note

the default value has changed to 2048 in kernel 4.6.

#+end_quote

*metadata_ratio=*/value/

(default: 0, internal logic)

Specifies that 1 metadata chunk should be allocated after every value data chunks. Default behaviour depends on internal logic, some percent of unused metadata space is attempted to be maintained but is not always possible if there’s not enough space left for chunk allocation. The option could be useful to override the internal logic in favor of the metadata allocation if the expected workload is supposed to be metadata intense (snapshots, reflinks, xattrs, inlined files).

norecovery

(since: 4.5, default: off)

Do not attempt any data recovery at mount time. This will disable logreplay and avoids other write operations. Note that this option is the same as nologreplay.

#+begin_quote

Note

The opposite option recovery used to have different meaning but was changed for consistency with other filesystems, where norecovery is used for skipping log replay. BTRFS does the same and in general will try to avoid any write operations.

#+end_quote

rescan_uuid_tree

(since: 3.12, default: off)

Force check and rebuild procedure of the UUID tree. This should not normally be needed.

rescue

(since: 5.9)

Modes allowing mount with damaged filesystem structures.

#+begin_quote ·

usebackuproot (since: 5.9, replaces standalone option usebackuproot)

·

nologreplay (since: 5.9, replaces standalone option nologreplay)

·

ignorebadroots, ibadroots (since: 5.11)

·

ignoredatacsums, idatacsums (since: 5.11)

·

all (since: 5.9)

#+end_quote

skip_balance

(since: 3.3, default: off)

Skip automatic resume of an interrupted balance operation. The operation can later be resumed with btrfs balance resume, or the paused state can be removed with btrfs balance cancel. The default behaviour is to resume an interrupted balance immediately after a volume is mounted.

space_cache, space_cache=*/version/, *nospace_cache

(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2 since 4.5, default: space_cache=v1)

Options to control the free space cache. The free space cache greatly improves performance when reading block group free space into memory. However, managing the space cache consumes some resources, including a small amount of disk space.

There are two implementations of the free space cache. The original one, referred to as v1, is the safe default. The v1 space cache can be disabled at mount time with nospace_cache without clearing.

On very large filesystems (many terabytes) and certain workloads, the performance of the v1 space cache may degrade drastically. The v2 implementation, which adds a new B-tree called the free space tree, addresses this issue. Once enabled, the v2 space cache will always be used and cannot be disabled unless it is cleared. Use clear_cache,space_cache=v1 or clear_cache,nospace_cache to do so. If v2 is enabled, kernels without v2 support will only be able to mount the filesystem in read-only mode.

The *btrfs-check*(8) and *mkfs.btrfs*(8) commands have full v2 free space cache support since v4.19.

If a version is not explicitly specified, the default implementation will be chosen, which is v1.

ssd, ssd_spread, nossd, nossd_spread

(default: SSD autodetected)

Options to control SSD allocation schemes. By default, BTRFS will enable or disable SSD optimizations depending on status of a device with respect to rotational or non-rotational type. This is determined by the contents of /sys/block/DEV/queue/rotational). If it is 0, the ssd option is turned on. The option nossd will disable the autodetection.

The optimizations make use of the absence of the seek penalty that’s inherent for the rotational devices. The blocks can be typically written faster and are not offloaded to separate threads.

#+begin_quote

Note

Since 4.14, the block layout optimizations have been dropped. This used to help with first generations of SSD devices. Their FTL (flash translation layer) was not effective and the optimization was supposed to improve the wear by better aligning blocks. This is no longer true with modern SSD devices and the optimization had no real benefit. Furthermore it caused increased fragmentation. The layout tuning has been kept intact for the option ssd_spread.

The ssd_spread mount option attempts to allocate into bigger and aligned chunks of unused space, and may perform better on low-end SSDs. ssd_spread implies ssd, enabling all other SSD heuristics as well. The option nossd will disable all SSD options while nossd_spread only disables ssd_spread. #+end_quote

*subvol=*/path/

Mount subvolume from path rather than the toplevel subvolume. The path is always treated as relative to the toplevel subvolume. This mount option overrides the default subvolume set for the given filesystem.

*subvolid=*/subvolid/

Mount subvolume specified by a subvolid number rather than the toplevel subvolume. You can use btrfs subvolume list of btrfs subvolume show to see subvolume ID numbers. This mount option overrides the default subvolume set for the given filesystem.

#+begin_quote

Note

if both subvolid and subvol are specified, they must point at the same subvolume, otherwise the mount will fail.

#+end_quote

*thread_pool=*/number/

(default: min(NRCPUS + 2, 8) )

The number of worker threads to start. NRCPUS is number of on-line CPUs detected at the time of mount. Small number leads to less parallelism in processing data and metadata, higher numbers could lead to a performance hit due to increased locking contention, process scheduling, cache-line bouncing or costly data transfers between local CPU memories.

treelog, notreelog

(default: on)

Enable the tree logging used for fsync and O_SYNC writes. The tree log stores changes without the need of a full filesystem sync. The log operations are flushed at sync and transaction commit. If the system crashes between two such syncs, the pending tree log operations are replayed during mount.

#+begin_quote

Warning

currently, the tree log is replayed even with a read-only mount! To disable that behaviour, also mount with nologreplay.

The tree log could contain new files/directories, these would not exist on a mounted filesystem if the log is not replayed. #+end_quote

usebackuproot

(since: 4.6, default: off)

Enable autorecovery attempts if a bad tree root is found at mount time. Currently this scans a backup list of several previous tree roots and tries to use the first readable. This can be used with read-only mounts as well.

#+begin_quote

Note

This option has replaced recovery.

#+end_quote

user_subvol_rm_allowed

(default: off)

Allow subvolumes to be deleted by their respective owner. Otherwise, only the root user can do that.

#+begin_quote

Note

historically, any user could create a snapshot even if he was not owner of the source subvolume, the subvolume deletion has been restricted for that reason. The subvolume creation has been restricted but this mount option is still required. This is a usability issue. Since 4.18, the *rmdir*(2) syscall can delete an empty subvolume just like an ordinary directory. Whether this is possible can be detected at runtime, see rmdir_subvol feature in FILESYSTEM FEATURES.

#+end_quote

The basic set of filesystem features gets extended over time. The backward compatibility is maintained and the features are optional, need to be explicitly asked for so accidental use will not create incompatibilities.

There are several classes and the respective tools to manage the features:

at mkfs time only

This is namely for core structures, like the b-tree nodesize or checksum algorithm, see *mkfs.btrfs*(8) for more details.

after mkfs, on an unmounted filesystem

Features that may optimize internal structures or add new structures to support new functionality, see btrfstune*(8). The command *btrfs inspect-internal dump-super device will dump a superblock, you can map the value of incompat_flags to the features listed below

after mkfs, on a mounted filesystem

The features of a filesystem (with a given UUID) are listed in sys/fs/btrfs/UUID/features, one file per feature. The status is stored inside the file. The value 1 is for enabled and active, while 0 means the feature was enabled at mount time but turned off afterwards.

Whether a particular feature can be turned on a mounted filesystem can be found in the directory sys/fs/btrfs/features, one file per feature. The value 1 means the feature can be enabled.

List of features (see also *mkfs.btrfs*(8) section FILESYSTEM FEATURES):

big_metadata

(since: 3.4)

the filesystem uses nodesize for metadata blocks, this can be bigger than the page size

compress_lzo

(since: 2.6.38)

the lzo compression has been used on the filesystem, either as a mount option or via btrfs filesystem defrag.

compress_zstd

(since: 4.14)

the zstd compression has been used on the filesystem, either as a mount option or via btrfs filesystem defrag.

default_subvol

(since: 2.6.34)

the default subvolume has been set on the filesystem

extended_iref

(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

free_space_tree

(since: 4.5)

free space representation using a dedicated b-tree, successor of v1 space cache

metadata_uuid

(since: 5.0)

the main filesystem UUID is the metadata_uuid, which stores the new UUID only in the superblock while all metadata blocks still have the UUID set at mkfs time, see *btrfstune*(8) for more

mixed_backref

(since: 2.6.31)

the last major disk format change, improved backreferences, now default

mixed_groups

(since: 2.6.37)

mixed data and metadata block groups, ie. the data and metadata are not separated and occupy the same block groups, this mode is suitable for small volumes as there are no constraints how the remaining space should be used (compared to the split mode, where empty metadata space cannot be used for data and vice versa)

on the other hand, the final layout is quite unpredictable and possibly highly fragmented, which means worse performance

no_holes

(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

raid1c34

(since: 5.5)

extended RAID1 mode with copies on 3 or 4 devices respectively

raid56

(since: 3.9)

the filesystem contains or contained a raid56 profile of block groups

rmdir_subvol

(since: 4.18)

indicate that *rmdir*(2) syscall can delete an empty subvolume just like an ordinary directory. Note that this feature only depends on the kernel version.

skinny_metadata

(since: 3.10)

reduced-size metadata for extent references, saves a few percent of metadata

send_stream_version

(since: 5.10)

number of the highest supported send stream version

supported_checksums

(since: 5.5)

list of checksum algorithms supported by the kernel module, the respective modules or built-in implementing the algorithms need to be present to mount the filesystem, see CHECKSUM ALGORITHMS

supported_sectorsizes

(since: 5.13)

list of values that are accepted as sector sizes (mkfs.btrfs –sectorsize) by the running kernel

supported_rescue_options

(since: 5.11)

list of values for the mount option rescue that are supported by the running kernel, see *btrfs*(5)

zoned

(since: 5.12)

zoned mode is allocation/write friendly to host-managed zoned devices, allocation space is partitioned into fixed-size zones that must be updated sequentially, see ZONED MODE

There are several checksum algorithms supported. The default and backward compatible is crc32c. Since kernel 5.5 there are three more with different characteristics and trade-offs regarding speed and strength. The following list may help you to decide which one to select.

CRC32C (32bit digest)

default, best backward compatibility, very fast, modern CPUs have instruction-level support, not collision-resistant but still good error detection capabilities

XXHASH (64bit digest)

can be used as CRC32C successor, very fast, optimized for modern CPUs utilizing instruction pipelining, good collision resistance and error detection

SHA256 (256bit digest)

a cryptographic-strength hash, relatively slow but with possible CPU instruction acceleration or specialized hardware cards, FIPS certified and in wide use

BLAKE2b (256bit digest)

a cryptographic-strength hash, relatively fast with possible CPU acceleration using SIMD extensions, not standardized but based on BLAKE which was a SHA3 finalist, in wide use, the algorithm used is BLAKE2b-256 that’s optimized for 64bit platforms

The digest size affects overall size of data block checksums stored in the filesystem. The metadata blocks have a fixed area up to 256bits (32 bytes), so there’s no increase. Each data block has a separate checksum stored, with additional overhead of the b-tree leaves.

Approximate relative performance of the algorithms, measured against CRC32C using reference software implementations on a 3.5GHz intel CPU:

Digest	Cycles/4KiB	Ratio	Implementation
CRC32C	1700	1.00	CPU instruction
XXHASH	2500	1.44	reference impl.
SHA256	105000	61	reference impl.
SHA256	36000	21	libgcrypt/AVX2
SHA256	63000	37	libsodium/AVX2
BLAKE2b	22000	13	reference impl.
BLAKE2b	19000	11	libgcrypt/AVX2
BLAKE2b	19000	11	libsodium/AVX2

Many kernels are configured with SHA256 as built-in and not as a module. The accelerated versions are however provided by the modules and must be loaded explicitly (modprobe sha256) before mounting the filesystem to make use of them. You can check in /sys/fs/btrfs/FSID/checksum which one is used. If you see sha256-generic, then you may want to unmount and mount the filesystem again, changing that on a mounted filesystem is not possible. Check the file /proc/crypto, when the implementation is built-in, you’d find

    name         : sha256
    driver       : sha256-generic
    module       : kernel
    priority     : 100
    ...

while accelerated implementation is e.g.

    name         : sha256
    driver       : sha256-avx2
    module       : sha256_ssse3
    priority     : 170
    ...

Btrfs supports transparent file compression. There are three algorithms available: ZLIB, LZO and ZSTD (since v4.14). Basically, compression is on a file by file basis. You can have a single btrfs mount point that has some files that are uncompressed, some that are compressed with LZO, some with ZLIB, for instance (though you may not want it that way, it is supported).

To enable compression, mount the filesystem with options compress or compress-force. Please refer to section MOUNT OPTIONS. Once compression is enabled, all new writes will be subject to compression. Some files may not compress very well, and these are typically not recompressed but still written uncompressed.

Each compression algorithm has different speed/ratio trade offs. The levels can be selected by a mount option and affect only the resulting size (ie. no compatibility issues).

Basic characteristics:

ZLIB	slower, higher compression ratio
	#+begin_quote
	·
	levels: 1 to 9, mapped directly, default level is 3
	#+end_quote
	#+begin_quote
	·
	good backward compatibility
	#+end_quote
LZO	faster compression and decompression than zlib, worse compression ratio, designed to be fast
	#+begin_quote
	·
	no levels
	#+end_quote
	#+begin_quote
	·
	good backward compatibility
	#+end_quote
ZSTD	compression comparable to zlib with higher compression/decompression speeds and different ratio
	#+begin_quote
	·
	levels: 1 to 15
	#+end_quote
	#+begin_quote
	·
	since 4.14, levels since 5.1
	#+end_quote

The differences depend on the actual data set and cannot be expressed by a single number or recommendation. Higher levels consume more CPU time and may not bring a significant improvement, lower levels are close to real time.

The algorithms could be mixed in one file as they’re stored per extent. The compression can be changed on a file by btrfs filesystem defrag command, using the -c option, or by btrfs property set using the compression property. Setting compression by chattr +c utility will set it to zlib.

There are several operations that affect the whole filesystem and cannot be run in parallel. Attempt to start one while another is running will fail.

Since kernel 5.10 the currently running operation can be obtained from /sys/fs/UUID/exclusive_operation with following values and operations:

·

balance

·

device add

·

device delete

·

device replace

·

resize

·

swapfile activate

·

none

Enqueuing is supported for several btrfs subcommands so they can be started at once and then serialized.

maximum file name length

255

maximum symlink target length

depends on the nodesize value, for 4k it’s 3949 bytes, for larger nodesize it’s 4095 due to the system limit PATH_MAX

The symlink target may not be a valid path, ie. the path name components can exceed the limits (NAME_MAX), there’s no content validation at *symlink*(3) creation.

maximum number of inodes

2^64 but depends on the available metadata space as the inodes are created dynamically

inode numbers

minimum number: 256 (for subvolumes), regular files and directories: 257

maximum file length

inherent limit of btrfs is 2^64 (16 EiB) but the linux VFS limit is 2^63 (8 EiB)

maximum number of subvolumes

the subvolume ids can go up to 2^64 but the number of actual subvolumes depends on the available metadata space, the space consumed by all subvolume metadata includes bookkeeping of shared extents can be large (MiB, GiB)

maximum number of hardlinks of a file in a directory

65536 when the extref feature is turned on during mkfs (default), roughly 100 otherwise

minimum filesystem size

the minimal size of each device depends on the mixed-bg feature, without that (the default) it’s about 109MiB, with mixed-bg it’s is 16MiB

GRUB2 (https://www.gnu.org/software/grub) has the most advanced support of booting from BTRFS with respect to features.

U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for booting but not all BTRFS features are implemented, check the documentation.

EXTLINUX (from the https://syslinux.org project) can boot but does not support all features. Please check the upstream documentation before you use it.

The first 1MiB on each device is unused with the exception of primary superblock that is on the offset 64KiB and spans 4KiB.

The btrfs filesystem supports setting file attributes or flags. Note there are old and new interfaces, with confusing names. The following list should clarify that:

·

attributes: *chattr*(1) or *lsattr*(1) utilities (the ioctls are FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the attributes are also called flags

·

xflags: to distinguish from the previous, it’s extended flags, with tunable bits similar to the attributes but extensible and new bits will be added in the future (the ioctls are FS_IOC_FSGETXATTR and FS_IOC_FSSETXATTR but they are not related to extended attributes that are also called xattrs), there’s no standard tool to change the bits, there’s support in xfs_io*(8) as command *xfs_io -c chattr

Since version 5.12 btrfs supports so called zoned mode. This is a special on-disk format and allocation/write strategy that’s friendly to zoned devices. In short, a device is partitioned into fixed-size zones and each zone can be updated by append-only manner, or reset. As btrfs has no fixed data structures, except the super blocks, the zoned mode only requires block placement that follows the device constraints. You can learn about the whole architecture at https://zonedstorage.io .

The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note that there are devices that appear as non-zoned but actually are, this is drive-managed and using zoned mode won’t help.

The zone size depends on the device, typical sizes are 256MiB or 1GiB. In general it must be a power of two. Emulated zoned devices like null_blk allow to set various zone sizes.

There’s a character special device /dev/btrfs-control with major and minor numbers 10 and 234 (the device can be found under the misc category).

    $ ls -l /dev/btrfs-control
    crw------- 1 root root 10, 234 Jan  1 12:00 /dev/btrfs-control

The device accepts some ioctl calls that can perform following actions on the filesystem module:

·

scan devices for btrfs filesystem (ie. to let multi-device filesystems mount automatically) and register them with the kernel module

·

similar to scan, but also wait until the device scanning process is finished for a given filesystem

·

get the supported features (can be also found under /sys/fs/btrfs/features)

The device is created when btrfs is initialized, either as a module or a built-in functionality and makes sense only in connection with that. Running eg. mkfs without the module loaded will not register the device and will probably warn about that.

In rare cases when the module is loaded but the device is not present (most likely accidentally deleted), it’s possible to recreate it by

    # mknod --mode=600 /dev/btrfs-control c 10 234

or (since 5.11) by a convenience command

    # btrfs rescue create-control-device

The control device is not strictly required but the device scanning will not work and a workaround would need to be used to mount a multi-device filesystem. The mount option device can trigger the device scanning during mount, see also btrfs device scan.

It is possible that a btrfs filesystem contains multiple block group profiles of the same type. This could happen when a profile conversion using balance filters is interrupted (see *btrfs-balance*(8)). Some btrfs commands perform a test to detect this kind of condition and print a warning like this:

    WARNING: Multiple block group profiles detected, see man btrfs(5).
    WARNING:   Data: single, raid1
    WARNING:   Metadata: single, raid1

The corresponding output of btrfs filesystem df might look like:

    WARNING: Multiple block group profiles detected, see man btrfs(5).
    WARNING:   Data: single, raid1
    WARNING:   Metadata: single, raid1
    Data, RAID1: total=832.00MiB, used=0.00B
    Data, single: total=1.63GiB, used=0.00B
    System, single: total=4.00MiB, used=16.00KiB
    Metadata, single: total=8.00MiB, used=112.00KiB
    Metadata, RAID1: total=64.00MiB, used=32.00KiB
    GlobalReserve, single: total=16.25MiB, used=0.00B

There’s more than one line for type Data and Metadata, while the profiles are single and RAID1.

This state of the filesystem OK but most likely needs the user/administrator to take an action and finish the interrupted tasks. This cannot be easily done automatically, also the user knows the expected final profiles.

In the example above, the filesystem started as a single device and single block group profile. Then another device was added, followed by balance with convert=raid1 but for some reason hasn’t finished. Restarting the balance with convert=raid1 will continue and end up with filesystem with all block group profiles RAID1.

Note

If you’re familiar with balance filters, you can use convert=raid1,profiles=single,soft, which will take only the unconverted single profiles and convert them to raid1. This may speed up the conversion as it would not try to rewrite the already convert raid1 profiles.

Having just one profile is desired as this also clearly defines the profile of newly allocated block groups, otherwise this depends on internal allocation policy. When there are multiple profiles present, the order of selection is RAID6, RAID5, RAID10, RAID1, RAID0 as long as the device number constraints are satisfied.

Commands that print the warning were chosen so they’re brought to user attention when the filesystem state is being changed in that regard. This is: device add, device delete, balance cancel, balance pause. Commands that report space usage: filesystem df, device usage. The command filesystem usage provides a line in the overall summary:

        Multiple profiles:                 yes (data, metadata)

The COW mechanism and multiple devices under one hood enable an interesting concept, called a seeding device: extending a read-only filesystem on a single device filesystem with another device that captures all writes. For example imagine an immutable golden image of an operating system enhanced with another device that allows to use the data from the golden image and normal operation. This idea originated on CD-ROMs with base OS and allowing to use them for live systems, but this became obsolete. There are technologies providing similar functionality, like unionmount, overlayfs or qcow2 image snapshot.

The seeding device starts as a normal filesystem, once the contents is ready, btrfstune -S 1 is used to flag it as a seeding device. Mounting such device will not allow any writes, except adding a new device by btrfs device add. Then the filesystem can be remounted as read-write.

Given that the filesystem on the seeding device is always recognized as read-only, it can be used to seed multiple filesystems, at the same time. The UUID that is normally attached to a device is automatically changed to a random UUID on each mount.

Once the seeding device is mounted, it needs the writable device. After adding it, something like remount -o remount,rw /path makes the filesystem at /path ready for use. The simplest usecase is to throw away all changes by unmounting the filesystem when convenient.

Alternatively, deleting the seeding device from the filesystem can turn it into a normal filesystem, provided that the writable device can also contain all the data from the seeding device.

The seeding device flag can be cleared again by btrfstune -f -s 0, eg. allowing to update with newer data but please note that this will invalidate all existing filesystems that use this particular seeding device. This works for some usecases, not for others, and a forcing flag to the command is mandatory to avoid accidental mistakes.

Example how to create and use one seeding device:

    # mkfs.btrfs /dev/sda
    # mount /dev/sda /mnt/mnt1
    # ... fill mnt1 with data
    # umount /mnt/mnt1
    # btrfstune -S 1 /dev/sda
    # mount /dev/sda /mnt/mnt1
    # btrfs device add /dev/sdb /mnt
    # mount -o remount,rw /mnt/mnt1
    # ... /mnt/mnt1 is now writable

Now /mnt/mnt1 can be used normally. The device /dev/sda can be mounted again with a another writable device:

    # mount /dev/sda /mnt/mnt2
    # btrfs device add /dev/sdc /mnt/mnt2
    # mount -o remount,rw /mnt/mnt2
    # ... /mnt/mnt2 is now writable

The writable device (/dev/sdb) can be decoupled from the seeding device and used independently:

    # btrfs device delete /dev/sda /mnt/mnt1

As the contents originated in the seeding device, it’s possible to turn /dev/sdb to a seeding device again and repeat the whole process.

A few things to note:

·

it’s recommended to use only single device for the seeding device, it works for multiple devices but the single profile must be used in order to make the seeding device deletion work

·

block group profiles single and dup support the usecases above

·

the label is copied from the seeding device and can be changed by btrfs filesystem label

·

each new mount of the seeding device gets a new random UUID

The RAID56 feature provides striping and parity over several devices, same as the traditional RAID5/6. There are some implementation and design deficiencies that make it unreliable for some corner cases and the feature should not be used in production, only for evaluation or testing. The power failure safety for metadata with RAID56 is not 100%.

/A storage model is a model that captures key physical aspects of data structure in a data store. A filesystem is the logical structure organizing data on top of the storage device./

The filesystem assumes several features or limitations of the storage device and utilizes them or applies measures to guarantee reliability. BTRFS in particular is based on a COW (copy on write) mode of writing, ie. not updating data in place but rather writing a new copy to a different location and then atomically switching the pointers.

In an ideal world, the device does what it promises. The filesystem assumes that this may not be true so additional mechanisms are applied to either detect misbehaving hardware or get valid data by other means. The devices may (and do) apply their own detection and repair mechanisms but we won’t assume any.

The following assumptions about storage devices are considered (sorted by importance, numbers are for further reference):

atomicity of reads and writes of blocks/sectors (the smallest unit of data the device presents to the upper layers)

there’s a flush command that instructs the device to forcibly order writes before and after the command; alternatively there’s a barrier command that facilitates the ordering but may not flush the data

data sent to write to a given device offset will be written without further changes to the data and to the offset

writes can be reordered by the device, unless explicitly serialized by the flush command

reads and writes can be freely reordered and interleaved

The consistency model of BTRFS builds on these assumptions. The logical data updates are grouped, into a generation, written on the device, serialized by the flush command and then the super block is written ending the generation. All logical links among metadata comprising a consistent view of the data may not cross the generation boundary.

The following is based on information publicly available, user feedback, community discussions or bug report analyses. It’s not complete and further research is encouraged when in doubt.

*acl*(5), *btrfs*(8), *chattr*(1), *fstrim*(8), *ioctl*(2), *mkfs.btrfs*(8), *mount*(8), *swapon*(8)