neulab/tldr huggingface dataset:
The to pattern is a filename with embedded wildcard indexes, where an index consists of the character '#' followed by a string of digits.  When a source file matches a from pattern, a target name for the file is constructed out of the to pattern by replacing the wildcard indexes by the actual characters that matched the referenced wildcards in the source name.  Thus, if the from pattern is "abc*.*" and the to pattern is "xyz#2.#1", then "abc.txt" is targeted to "xyztxt.".  (The first '*' matched "", and the second matched "txt".)  Similarly, for the pattern pair ";*.[clp]" -> "#1#3/#2", "foo1/foo2/prog.c" is targeted to "foo1/foo2/c/prog".  Note that there is no '/' following the "#1" in the to pattern, since the string matched by any ';' is always either empty or ends in a '/'.  In this case, it matches "foo1/foo2/".
To convert the string matched by a wildcard to either lowercase or uppercase before embedding it in the target name, insert 'l' or 'u', respectively, between the '#' and the string of digits.
The to pattern, like the from pattern, can begin with a "~/" (see above). This does not necessitate enclosing the to pattern in quotes on the command line since csh(1) expands the '~' in the exact same manner as mmv (or, in the case of sh(1), does not expand it at all).
For all task options other than -r, if the target name is a directory, the real target name is formed by appending a '/' followed by the last component of the source file name.  For example, "mmv dir1/a dir2" will, if "dir2" is indeed a directory, actually move "dir1/a" to "dir2/a". However, if "dir2/a" already exists and is itself a directory, this is considered an error.
To strip any character (e.g. '*', '?', or '#') of its special meaning to mmv, as when the actual replacement name must contain the character '#', precede the special character with a Â´\' (and enclose the argument in quotes because of the shell).  This also works to terminate a wildcard index when it has to be followed by a digit in the filename, e.g. "a#1\1".
Chains and Cycles
A chain is a sequence of specified actions where the target name of one action refers to the source file of another action.  For example,
mmv a b b c
specifies the chain "a" -> "b" -> "c".  A cycle is a chain where the last target name refers back to the first source file, e.g. "mmv a a".  Mmv detects chains and cycles regardless of the order in which their constituent actions are actually given.  Where allowed, i.e. in moving, renaming, and appending files, chains and cycles are handled gracefully, by performing them in the proper order.  Cycles are broken by first renaming one of the files to a temporary name (or just remembering its original size when doing appends).
Collisions and Deletions
When any two or more matching files would have to be moved, copied, or linked to the same target filename, mmv detects the condition as an error before performing any actions.  Furthermore, mmv checks if any of its actions will result in the destruction of existing files.  If the -d (delete) option is specified, all file deletions or overwrites are done silently.  Under -p (protect), all deletions or overwrites (except those specified with "(*)" on the standard input, see below) are treated as errors.  And if neither option is specified, the user is queried about each deletion or overwrite separately.  (A new stream to "/dev/tty" is used for all interactive queries, not the standard input.)
Whenever any error in the user's action specifications is detected, an error message is given on the standard output, and mmv proceeds to check the rest of the specified actions.  Once all errors are detected, mmv queries the user whether he wishes to continue by avoiding the erroneous actions or to abort altogether.  This and all other queries may be avoided by specifying either the -g (go) or -t (terminate) option.  The former will resolve all difficulties by avoiding the erroneous actions; the latter will abort mmv if any errors are detected.  Specifying either of them defaults mmv to -p, unless -d is specified (see above).  Thus, -g and -t are most useful when running mmv in the background or in a shell script, when interactive queries are undesirable.
Once the actions to be performed are determined, mmv performs them silently, unless either the -v (verbose) or -n (no-execute) option is specified.  The former causes mmv to report each performed action on the standard output as
a -> b : done.
Here, "a" and "b" would be replaced by the source and target names, respectively.  If the action deletes the old target, a "(*)" is inserted after the the target name.  Also, the "->" symbol is modified when a cycle has to be broken: the '>' is changed to a '^' on the action prior to which the old target is renamed to a temporary, and the '-' is changed to a '=' on the action where the temporary is used.
Under -n, none of the actions are performed, but messages like the above are printed on the standard output with the ": done." omitted.
The output generated by -n can (after editing, if desired) be fed back to mmv on the standard input (by omitting the from -- to pair on the mmv command line).  To facilitate this, mmv ignores lines on the standard input that look like its own error and "done" messages, as well as all lines beginning with white space, and will accept pattern pairs with or without the intervening "->" (or "-^", "=>", or "=^").  Lines with "(*)" after the target pattern have the effect of enabling -d for the files matching this pattern only, so that such deletions are done silently.
WARNING: This means that unexpected things may happen if files matched by the patterns contain spaces. See http://bugs.debian.org/149873 for details.
When feeding mmv its own output, one must remember to specify again the task option (if any) originally used to generate it.
Although mmv attempts to predict all mishaps prior to performing any specified actions, accidents may happen.  For example, mmv does not check for adequate free space when copying.  Thus, despite all efforts, it is still possible for an action to fail after some others have already been done.  To make recovery as easy as possible, mmv reports which actions have already been done and which are still to be performed after such a failure occurs.  It then aborts, not attempting to do anything else. Once the user has cleared up the problem, he can feed this report back to mmv on the standard input to have it complete the task.  (The user is queried for a file name to dump this report if the standard output has not been redirected.)
Mmv exits with status 1 if it aborts before doing anything, with status 2 if it aborts due to failure after completing some of the actions, and with status 0 otherwise.
mv(1), cp(1), ln(1), umask(1)
Vladimir Lanin lanin@csd2.nyu.edu
If the search pattern is not quoted, the shell expands the wildcards. Mmv then (usually) gives some error message, but can not determine that the lack of quotes is the cause.
To avoid difficulties in semantics and error checking, mmv refuses to move or create directories.
BTRFS-MAN5(5)                     Btrfs Manual                     BTRFS-MAN5(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:
1. mount options
2. filesystem features
3. checksum algorithms
4. compression
5. filesystem exclusive operations
6. filesystem limits
7. bootloader support
8. file attributes
9. zoned mode
10. control device
11. filesystems with multiple block group profiles
12. seeding device
13. raid56 status and recommended practices
14. storage model
15. 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.
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.
Note If compression is enabled, nodatacow and nodatasum are disabled.
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)).
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).
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)).
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.
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.
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.
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 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.
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.
Warning currently, the tree log is replayed even with a read-only mount! To disable that behaviour, mount also with nologreplay.
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.
Note the default value has changed to 2048 in kernel 4.6.
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.
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.
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.
•   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)
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.
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.
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.
Note if both subvolid and subvol are specified, they must point at the same subvolume, otherwise the mount will fail.
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.
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.
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.
Note This option has replaced recovery.
user_subvol_rm_allowed (default: off)
Allow subvolumes to be deleted by their respective owner. Otherwise, only the root user can do that.
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.
DEPRECATED MOUNT OPTIONS List of mount options that have been removed, kept for backward compatibility.
recovery (since: 3.2, default: off, deprecated since: 4.5)
Note this option has been replaced by usebackuproot and should not be used but will work on 4.5+ kernels.
inode_cache, noinode_cache (removed in: 5.11, since: 3.0, default: off)
Note the functionality has been removed in 5.11, any stale data created by previous use of the inode_cache option can be removed by btrfs check --clear-ino-cache.
NOTES ON GENERIC MOUNT OPTIONS Some of the general mount options from mount(8) that affect BTRFS and are worth mentioning.
noatime under read intensive work-loads, specifying noatime significantly improves performance because no new access time information needs to be written. Without this option, the default is relatime, which only reduces the number of inode atime updates in comparison to the traditional strictatime. The worst case for atime updates under relatime occurs when many files are read whose atime is older than 24 h and which are freshly snapshotted. In that case the atime is updated and COW happens - for each file - in bulk. See also https://lwn.net/Articles/499293/ - Atime and btrfs: a bad combination? (LWN, 2012-05-31).
Note that noatime may break applications that rely on atime uptimes like the venerable Mutt (unless you use maildir mailboxes).
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
SWAPFILE SUPPORT The swapfile is supported since kernel 5.0. Use swapon(8) to activate the swapfile. There are some limitations of the implementation in btrfs and linux swap subsystem:
•   filesystem - must be only single device
•   filesystem - must have only single data profile
•   swapfile - the containing subvolume cannot be snapshotted
•   swapfile - must be preallocated
•   swapfile - must be nodatacow (ie. also nodatasum)
•   swapfile - must not be compressed
The limitations come namely from the COW-based design and mapping layer of blocks that allows the advanced features like relocation and multi-device filesystems. However, the swap subsystem expects simpler mapping and no background changes of the file blocks once they’ve been attached to swap.
With active swapfiles, the following whole-filesystem operations will skip swapfile extents or may fail:
•   balance - block groups with swapfile extents are skipped and reported, the rest will be processed normally
•   resize grow - unaffected
•   resize shrink - works as long as the extents are outside of the shrunk range
•   device add - a new device does not interfere with existing swapfile and this operation will work, though no new swapfile can be activated afterwards
•   device delete - if the device has been added as above, it can be also deleted
•   device replace - ditto
When there are no active swapfiles and a whole-filesystem exclusive operation is running (ie. balance, device delete, shrink), the swapfiles cannot be temporarily activated. The operation must finish first.
To create and activate a swapfile run the following commands:
# truncate -s 0 swapfile # chattr +C swapfile # fallocate -l 2G swapfile # chmod 0600 swapfile # mkswap swapfile # swapon swapfile
Please note that the UUID returned by the mkswap utility identifies the swap "filesystem" and because it’s stored in a file, it’s not generally visible and usable as an identifier unlike if it was on a block device.
The file will appear in /proc/swaps:
# cat /proc/swaps Filename          Type          Size           Used      Priority /path/swapfile    file          2097152        0         -2
The swapfile can be created as one-time operation or, once properly created, activated on each boot by the swapon -a command (usually started by the service manager). Add the following entry to /etc/fstab, assuming the filesystem that provides the /path has been already mounted at this point. Additional mount options relevant for the swapfile can be set too (like priority, not the btrfs mount options).
/path/swapfile        none        swap        defaults      0 0
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
•   levels: 1 to 9, mapped directly, default level is 3
•   good backward compatibility
LZO    faster compression and decompression than zlib, worse compression ratio, designed to be fast
•   no levels
ZSTD   compression comparable to zlib with higher compression/decompression speeds and different ratio
•   levels: 1 to 15
•   since 4.14, levels since 5.1
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.
INCOMPRESSIBLE DATA Files with already compressed data or with data that won’t compress well with the CPU and memory constraints of the kernel implementations are using a simple decision logic. If the first portion of data being compressed is not smaller than the original, the compression of the file is disabled — unless the filesystem is mounted with compress-force. In that case compression will always be attempted on the file only to be later discarded. This is not optimal and subject to optimizations and further development.
If a file is identified as incompressible, a flag is set (NOCOMPRESS) and it’s sticky. On that file compression won’t be performed unless forced. The flag can be also set by chattr +m (since e2fsprogs 1.46.2) or by properties with value no or none. Empty value will reset it to the default that’s currently applicable on the mounted filesystem.
There are two ways to detect incompressible data:
•   actual compression attempt - data are compressed, if the result is not smaller, it’s discarded, so this depends on the algorithm and level
•   pre-compression heuristics - a quick statistical evaluation on the data is peformed and based on the result either compression is performed or skipped, the NOCOMPRESS bit is not set just by the heuristic, only if the compression algorithm does not make an improvent
PRE-COMPRESSION HEURISTICS The heuristics aim to do a few quick statistical tests on the compressed data in order to avoid probably costly compression that would turn out to be inefficient. Compression algorithms could have internal detection of incompressible data too but this leads to more overhead as the compression is done in another thread and has to write the data anyway. The heuristic is read-only and can utilize cached memory.
The tests performed based on the following: data sampling, long repated pattern detection, byte frequency, Shannon entropy.
COMPATIBILITY WITH OTHER FEATURES Compression is done using the COW mechanism so it’s incompatible with nodatacow. Direct IO works on compressed files but will fall back to buffered writes. Currently nodatasum and compression don’t work together.
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
ATTRIBUTES a append only, new writes are always written at the end of the file
A no atime updates
c compress data, all data written after this attribute is set will be compressed. Please note that compression is also affected by the mount options or the parent directory attributes.
When set on a directory, all newly created files will inherit this attribute. This attribute cannot be set with m at the same time.
C no copy-on-write, file data modifications are done in-place
When set on a directory, all newly created files will inherit this attribute.
Note due to implementation limitations, this flag can be set/unset only on empty files.
d no dump, makes sense with 3rd party tools like dump(8), on BTRFS the attribute can be set/unset but no other special handling is done
D synchronous directory updates, for more details search open(2) for O_SYNC and O_DSYNC
i immutable, no file data and metadata changes allowed even to the root user as long as this attribute is set (obviously the exception is unsetting the attribute)
m no compression, permanently turn off compression on the given file. Any compression mount options will not affect this file. (chattr support added in 1.46.2)
When set on a directory, all newly created files will inherit this attribute. This attribute cannot be set with c at the same time.
S synchronous updates, for more details search open(2) for O_SYNC and O_DSYNC
No other attributes are supported. For the complete list please refer to the chattr(1) manual page.
XFLAGS There’s overlap of letters assigned to the bits with the attributes, this list refers to what xfs_io(8) provides:
i immutable, same as the attribute
a append only, same as the attribute
s synchronous updates, same as the attribute S
A no atime updates, same as the attribute
d no dump, same as the attribute
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.
REQUIREMENTS, LIMITATIONS •   all devices must have the same zone size
•   maximum zone size is 8GiB
•   mixing zoned and non-zoned devices is possible, the zone writes are emulated, but this is namely for testing
•   the super block is handled in a special way and is at different locations than on a non-zoned filesystem:
•   primary: 0B (and the next two zones)
•   secondary: 512G (and the next two zones)
•   tertiary: 4TiB (4096GiB, and the next two zones)
INCOMPATIBLE FEATURES The main constraint of the zoned devices is lack of in-place update of the data. This is inherently incompatbile with some features:
•   nodatacow - overwrite in-place, cannot create such files
•   fallocate - preallocating space for in-place first write
•   mixed-bg - unordered writes to data and metadata, fixing that means using separate data and metadata block groups
•   booting - the zone at offset 0 contains superblock, resetting the zone would destroy the bootloader data
Initial support lacks some features but they’re planned:
•   only single profile is supported
•   fstrim - due to dependency on free space cache v1