Feature-by-version.rst

Changes (feature/version)

Major features or significant feature enhancements by kernel version. For more information look below.

The version states at which version a feature has been merged into the mainline kernel. It does not tell anything about at which kernel version it is considered mature enough for production use. For an estimation on stability of features see :doc:`Status<Status>` page.

6.x

6.0 - send protocol v2

Send protocol update that adds new commands and extends existing functionality to write large data chunks. Compressed (and encrypted) extents can be optionally emitted and transferred as-is without the need to re-compress (or re-encrypt) on the receiving side.

6.0 - sysfs exports commit stats

The file :file:`/sys/fs/btrfs/FSID/commit_stats` shows number of commits and various time related statistics.

6.0 - sysfs exports chunk sizes

Chunk size value can be read from :file:`/sys/fs/btrfs/FSID/allocation/PROFILE/chunk_size`.

6.0 - sysfs shows zoned mode among features

The zoned mode has been supported since 5.10 and adding functionality. Now it's advertised among features.

6.0 - checksum implementation is logged at mount time

When a filesystem is mounted the implementation backing the checksums is logged. The information is also accessible in :file:`/sys/fs/btrfs/FSID/checksum`.

6.1 - sysfs support to temporarily skip exact qgroup accounting

Allow user override of qgroup accounting and make it temporarily out of date e.g. in case when there are several subvolumes deleted and the qgroup numbers need to be updated at some cost, an update after that can amortize the costs.

6.1 - scrub also repairs superblock

An improvement to scrub in case the superblock is detected to be corrupted, the repair happens immediately. Previously it was delayed until the next transaction commit for performance reasons that would store an updated and correct copy eventually.

6.1 - block group tree

An incompatible change that has to be enabled at mkfs time. Add a new b-tree item that stores information about block groups in a compact way that significantly improves mount time that's usually long due to fragmentation and scattered b-tree items tracking the individual block groups. Requires and also enables the free-space-tree and no-holes features.

6.1 - discard stats available in sysfs

The directory :file:`/sys/fs/btrfs/FSID/discard` exports statistics and tunables related to discard.

6.1 - additional qgroup stats in sysfs

The overall status of qgroups are exported in :file:`/sys/sys/fs/btrfs/FSID/qgroups/`.

6.1 - check that super block is unchanged at thaw time

Do full check of super block once a filesystem is thawed. This namely happens when system resumes from suspend or hibernation. Accidental change by other operating systems will be detected.

6.2 - discard=async on by default

Devices that support trim/discard will enable the asynchronous discard for the whole filesystem.

6.3 - discard=async settings tuned

The default IOPS limit has changed from 100 to 1000 and writing value 0 to :file:`/sys/fs/btrfs/FSID/discard/iops_limit` newly means to not do any throttling.

6.3 - block group allocation class heuristics

Pack files by size (up to 128k, up to 8M, more) to avoid fragmentation in block groups, assuming that file size and life time is correlated, in particular this may help during balance. The stats about the number of used classes per block group type is exported in :file:`/sys/fs/btrfs/FSID/allocation/\*/size_classes`.

6.3 - in DEV_INFO ioctl export per-device FSID

A seeding device could have a different FSID, available in sysfs and now available via DEV_INFO ioctl.

6.3 - send utimes cache, reduced stream size

Utimes for directories are emitted into the send stream only when finalizing the directory, the cache also gains significant speedups (up to 10x).

6.7 - raid-stripe-tree

New tree for logical mapping, allows some RAID modes for zoned mode.

6.7 - simplified quota accounting

A simplified mode of qgroups accounting

6.7 - temporary fsid

Mount of cloned devices is now possible, the filesystem will get a new randomly generated UUID on mount

6.8 - new mount API

Use new mount API (https://lwn.net/Articles/753473/)

6.9 - statx can read the subvolume id

The extendable syscall statx also returns the subvolume id and sets the result_mask bit STATX_SUBVOL.

6.9 - reflinked file range and concurrent read

An optimization for concurrent access to a range that is reflinked and read at the same time, the read latency is decreased due to reduced locking.

6.10 - automatic stale subvolume group removal

This applies to 0 level qgroups (the one automatically created for a subvolume), once the subvolume is deleted the respective qgroup is also deleted. This may take some time until the qgroup accounting is correct and consistent again as the subvolume deletion is delayed.

This is also affected by presence of the subvolume qgroup in higher level qgroups or the sysfs setting of drop_subtree_threshold that will need a quota rescan.

6.10 - sysfs reports reclaim status

A per-filesystem report of background reclaim status, file names matching reclaim_ in the space info directory.

6.10 - tunable dynamic background reclaim threshold

Run background block group reclaim (using the relocation/balance mechanism) if the used size is above the configured value and the dynamic reclaim is enabled (not by default). When enabled, there's a heuristic that ties to avoid increasing system load if there's enough unallocated space but will try hard (but cannot be perfect) to avoid a situation when there's last chunk remaining to make the relocation possible.

6.10 - new mount option rescue= mode ignoremetacsums

When enabled, any metadata checksum mismatch is ignored (in read-only mount), this may be useful in an interrupted checksum type conversion (:doc:`btrfstune`).

6.10 - new mount option rescue= mode ignoresuperflags

An option to ignore unknown super block flags, at this point applies only to the interrupted checksum conversion, but can be useful for similar operations in the future.

6.10 - tree-checker updates

Properly verify all types of directory items and reject unknown ones. Do relevant device item checks.

6.10 - allow to clone/reflink the tail extent

Check if the last inode extent (not a full block length) can be cloned and do it, this fixes a problem in send/receive.

6.10 - unlink updates ctime

Mandated by POSIX (https://pubs.opengroup.org/onlinepubs/9699919799/functions/unlink.html), the link count is changed.

6.11 - unconditionally wake up cleaner thread on SYNC ioctl

Avoid indirection when the BTRFS_IOC_SYNC ioctl is called and wake up the cleaner thread which is among other things responsible to clean deleted subvolumes.

6.11 - reduced locking around buffered reads

Improve concurrency by reducing scope of locking around buffered reads. The direct io is still locked but this should not be mixed with buffered writes.

6.12 - cancellable discard/TRIM

Add more points where the discard can be interrupted by signals before it finishes the whole operation.

6.13 - new config option CONFIG_BTRFS_EXPERIMENTAL

Add separate config option to distinguish purely debugging features (like extended safety checks) and features that still need some refinements (and were hidden under the debugging config option not to expose them to users). When enabled this namely covers extent tree v2, raid stripe tree, send protocol version 3 and checksum offloading strategy.

6.13 - encoded read integration with io_uring

The io_uring subsystem understands a command that is directed to Btrfs encoded read ioctl.

6.13 - new ioctl to wait for cleaned subvolumes

Add specialized ioctl to wait for deleted (and maybe not yet cleaned) subvolumes, available to any user. The related command :command:`btrfs subvolume sync` uses the privileged SEARCH_TREE ioctl otherwise.

6.13 - seeding device use case change

The sprout device (the writable one added to the seeding device) does not touch the superblock read-only status, preventing removal of accumulated deleted snapshots to be cleaned.

6.13 - update tree-checker to detect more wrong inline extent references

5.x

5.0 - swapfile

With some limitations where COW design does not work well with the swap implementation (nodatacow file, no compression, cannot be snapshotted, not possible on multiple devices, ...), as this is the most restricted but working setup, we'll try to improve that in the future

5.0 - metadata uuid

An optional incompat feature to assign a new filesystem UUID without overwriting all metadata blocks, stored only in superblock, unlike what :command:`btrfstune -u`

5.1 - FORGET_DEV ioctl

Unregister devices previously added by the scan ioctl, same effect as if the kernel module is reloaded.

5.1 - ZSTD level

Allow to set the ZSTD compression level via mount option, e.g. like compress=zstd:9. The levels match the default ZSTD compression levels. The default is 3, maximum is 15.

5.2 - pre-write checks

Verify metadata blocks before submitting them to the devices. This can catch consistency problems or bitflips.

5.5 - more checksums

New checksum algorithms: xxhash (64b), SHA256 (256b), BLAKE2b (256b).

5.5 - RAID1C34

RAID1 with 3- and 4- copies (over all devices).

5.6 - async discard

Mode of discard (mount -o discard=async) that merges freed extents to larger chunks and submits them for discard in a less intrusive way

5.6 - device info in sysfs

More information about device state can be found in per-filesystem sysfs directory.

5.7 - reflink/clone works on inline files

Inline files can be reflinked to the tail extent of other files

5.7 - faster balance cancel

More cancellation points in balance that will shorten the time to stop processing once :command:`btrfs balance cancel` is called.

5.7 - removed flag BTRFS_SUBVOL_CREATE_ASYNC

Remove support of flag BTRFS_SUBVOL_CREATE_ASYNC from subvolume creation ioctl.

5.7 - v2 of snapshot deletion ioctl

New ioctl BTRFS_IOC_SNAP_DESTROY_V2, deletion by subvolume id is now possible.

5.9 - mount option rescue

Unified mount option for actions that may help to access a damaged filesystem. Now supports: nologreplay, usebackuproot

5.9 - qgroups in sysfs

The information about qgroup status and relations is exported in :file:`/sys/fs/UUID/qgroups`

5.9 - FS_INFO ioctl

Export more information: checksum type, checksum size, generation, metadata_uuid

5.10 - exclusive ops in sysfs

Export which filesystem exclusive operation is running (balance, resize, device add/delete/replace, ...)

5.11 - remove inode_cache

Remove inode number caching feature (mount -o inode_cache)

5.11 - more rescue= modes

Additional modes for mount option rescue=: ignorebadroots/ibadroots, ignoredatacsums/idatacsums. All are exported in :file:`/sys/fs/btrfs/features/supported_rescue_options`.

5.12 - zoned mode

Support for zoned devices with special allocation/write mode to fixed-size zones. See :doc:`Zoned<Zoned-mode>`.

5.13 - supported_sectorsizes in sysfs

List supported sector sizes in sysfs file :file:`/sys/fs/btrfs/features/supported_sectorsizes`.

5.14 - sysfs scrub bw limit

Tunable bandwidth limit :file:`/sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max` for scrub (and device replace) for a given device.

5.14 - sysfs device stats

The device stats can be also found in :file:`/sys/fs/btrfs/FSID/devinfo/DEVID/error_stats`.

5.14 - cancellable resize, device delete

The filesystem resize and device delete operations can be cancelled by specifying cancel as the device name.

5.14 - property value reset

Change how empty value is interpreted. New behaviour will delete the value and reset it to default. This affects btrfs.compression where value no sets NOCOMPRESS bit while empty value resets all compression settings (either compression or NOCOMPRESS bit).

5.15 - fsverity

The fs-verity is a support layer that filesystems can hook into to support transparent integrity and authenticity protection of read-only files. https://www.kernel.org/doc/html/latest/filesystems/fsverity.html

5.15 - idmapped mount

Support mount with UID/GID mapped according to another namespace. https://lwn.net/Articles/837566/

5.16 - ZNS in zoned

Zoned namespaces. https://zonedstorage.io/docs/introduction/zns , https://lwn.net/Articles/865988/

5.17 - send and relocation

Send and relocation (balance, device remove, shrink, block group reclaim) can now work in parallel.

5.17 - device add vs balance

It is possible to add a device with paused balance.

Note

Since kernel 5.17.7 and btrfs-progs 5.17.1

5.17 - no warning with flushoncommit

Mounting with -o flushoncommit does not trigger the (harmless) warning at each transaction commit.

Note

Also backported to 5.15.27 and 5.16.13

5.18 - zoned and DUP metadata

DUP metadata works with zoned mode.

5.18 - encoded data ioctl

New ioctls to read and write pre-encoded data (i.e. no transformation and directly written as extents), now works for compressed data.

5.18 - removed balance ioctl v1

The support for ioctl BTRFS_IOC_BALANCE has been removed, superseded by BTRFS_IOC_BALANCE_V2 long time ago.

5.18 - cross-mount reflink works

The VFS limitation to reflink files on separate subvolume mounts of the same filesystem has been removed.

5.18 - syslog error messages with filesystem state

Messages are printed with a one letter tag ("state: X") that denotes in which state the filesystem was at this point:

• A - transaction aborted (permanent)
• E - filesystem error (permanent)
• M - remount in progress (transient)
• R - device replace in progress (transient)
• C - checksum checks disabled by mount option (rescue=ignoredatacsums)
• L - log tree replay did not complete due to some error

5.18 - tree-checker verifies transaction id pre-write

Metadata buffer to be written gets an extra check if the stored transaction number matches the current state of the filesystem.

5.19 - subpage support pages > 4KiB

Metadata node size is supported regardless of the CPU page size (minimum size is 4KiB), data sector size is supported <= page size. Additionally subpage also supports RAID56.

5.19 - per-type background threshold for reclaim

Add sysfs tunable for background reclaim threshold for all block group types (data, metadata, system).

5.19 - automatically repair device number mismatch

Device information is stored in two places, the number in the super block and items in the device tree. When this is goes out of sync, e.g. by device removal short before unmount, the next mount could fail. The b-tree is an authoritative information an can be used to override the stale value in the superblock.

5.19 - defrag can convert inline files to regular ones

The logic has been changed so that inline files are considered for defragmentation even if the mount option max_inline would prevent that. No defragmentation might happen but the inlined files are not skipped.

5.19 - explicit minimum zone size is 4MiB

Set the minimum limit of zone on zoned devices to 4MiB. Real devices zones are much larger, this is for emulated devices.

5.19 - sysfs tunable for automatic block group reclaim

Add possibility to set a threshold to automatically reclaim block groups also in non-zoned mode. By default completely empty block groups are reclaimed automatically but the threshold can be tuned in :file:`/sys/fs/btrfs/FSID/allocation/PROFILE/bg_reclaim_threshold`.

5.19 - tree-checker verifies metadata block ownership

Additional check done by tree-checker to verify relationship between a tree block and it's tree root owner.

4.x

4.0 - store otime

Save creation time (otime) for all new files and directories. For future use, current tool cannot read it directly.

4.2 - rootid ioctl accessible

The INO_LOOKUP will return root id (id of the containing subvolume), unrestricted and to all users if the treeid is 0.

4.2 - dedupe possible on the same inode

The EXTENT_SAME ioctl will accept the same inode as source and destination (ranges must not overlap).

4.3 - trim all free space

Trim will be performed also on the space that's not allocated by the chunks, not only free space within the allocated chunks.

4.4 - balance filter updates

Enhanced syntax and new balance filters:

• limit=min..max
• usage=min..max
• stripes=min..max

4.5 - free space tree

Improved implementation of free space cache (aka v2), using b-trees.

Note

Default since btrfs-progs 5.15, Kernel 4.9 fixes endianness bugs on big-endian machines, x86* is ok

4.5 - balance filter updates

Conversion to data/DUP profile possible through balance filters -- on single-device filesystem.

Note

mkfs.btrfs allows creating DUP on single device in the non-mixed mode since 4.4

4.6 - max_inline default

The default value of max_inline changed to 2048.

4.6 - read features from control device

The existing ioctl GET_SUPPORTED_FEATURES can be now used on the control device (:file:`/dev/btrfs-control`) and returns the supported features without any mounted filesystem.

4.7 - delete device by id

Add new ioctl RM_DEV_V2, pass device to be deleted by its ID.

4.7 - more renameat2 modes

Add support for RENAME_EXCHANGE and RENAME_WHITEOUT to renameat2 syscall. This also means that overlayfs is now supported on top of btrfs.

4.7 - balance filter updates

Conversion to data/DUP profile possible through balance filters -- on multiple-device filesystems.

Note

mkfs.btrfs allows creating DUP on multiple devices since 4.5.1

4.12 - RAID56: auto repair

Scrub will attempt auto-repair (similar to raid1/raid10)

4.13 - statx

Support for the enhanced statx syscall; file creation timestamp

4.13 - sysfs qgroups override

qgroups: new sysfs control file to allow temporary quota override with CAP_SYS_RESOURCE

4.13 - deprecated mount option alloc_start

That was a debugging helper, not used and not supposed to be used nowadays.

4.14 - ZSTD compression

New compression algorithm ZSTD, supposedly better ratio/speed performance.

4.14 - improved degraded mount

Allow degraded mount based on the chunk constraints, not device number constraints. E.g. when one device is missing but the remaining one holds all single chunks.

4.14 - deprecated user transaction ioctl

BTRFS_IOC_TRANS_START and BTRFS_IOC_TRANS_END, no known users, tricky to use; scheduled to be removed in 4.17

4.14 - refine SSD optimizations

The mount option ssd does not make any assumptions about block layout or management by the device anymore, leaving only the speedups based on low seek cost active. This could avoid some corner cases leading to excessive fragmentation. https://git.kernel.org/linus/583b723151794e2ff1691f1510b4e43710293875 The story so far.

4.15 - overlayfs

Overlayfs can now use btrfs as the lower filesystem.

4.15 - ref-verify

Debugging functionality to verify extent references. New mount option ref-verify, must be built with CONFIG_BTRFS_FS_REF_VERIFY.

4.15 - ZLIB level

Allow to set the ZLIB compression level via mount option, e.g. like compress=zlib:9. The levels match the default ZLIB compression levels. The default is 3.

4.15 - v2 of LOGICAL_INO ioctl

An enhanced version of ioctl that can translate logical extent offset to inode numbers, "who owns this block". For certain use cases the V1 performs bad and this is addressed by V2. See for more https://git.kernel.org/linus/d24a67b2d997c860a42516076f3315c2ad2d2884 .

4.15 - compression heuristics

Apply a few heuristics to the data before they're compressed to decide if it's likely to gain any space savings. The methods: frequency sampling, repeated pattern detection, Shannon entropy calculation.

4.16 - fallocate: zero range

Mode of the fallocate syscall to zero file range.

4.17 - removed user transaction ioctl

Deprecated in 4.14, see above.

4.17 - rmdir on subvolumes

Allow rmdir to delete an empty subvolume.

4.18 - XFLAGS ioctl

Add support for ioctl FS_IOC_FSSETXATTR/FS_IOC_FSGETXATTR, successor of FS_IOC_SETFLAGS/FS_IOC_GETFLAGS ioctl. Currently supports: APPEND, IMMUTABLE, NOATIME, NODUMP, SYNC. Note that the naming is very confusing, though it's named xattr, it does not mean the extended attributes. It should be referenced as extended inode flags or xflags.

4.18 - EXTENT_SAME ioctl / 16MiB chunks

The range for out-of-band deduplication implemented by the EXTENT_SAME ioctl will split the range into 16MiB chunks. Up to now this was the overall limit and effectively only the first 16MiB was deduplicated.

4.18 - GET_SUBVOL_INFO ioctl

New ioctl to read subvolume information (id, directory name, generation, flags, UUIDs, time). This does not require root permissions, only the regular access to to the subvolume.

4.18 - GET_SUBVOL_ROOTREF ioctl

New ioctl to enumerate subvolume references of a given subvolume. This does not require root permissions, only the regular access to to the subvolume.

4.18 - INO_LOOKUP_USER ioctl

New ioctl to lookup path by inode number. This does not require root permissions, only the regular access to to the subvolume, unlike the INO_LOOKUP ioctl.

4.19 - defrag ro/rw

Allow to run defrag on files that are normally accessible for read-write, but are currently opened in read-only mode.

3.x

3.0 - scrub

Read all data and verify checksums, repair if possible.

3.2 - auto raid repair

Automatic repair of broken data from a good copy

3.2 - root backups

Save a few previous versions of the most important tree roots at commit time, used by -o recovery

3.3 - integrity checker

Optional infrastructure to verify integrity of written metadata blocks

3.3 - backref walking

Groundwork to allow tracking owner of blocks, used via inspect-internal

3.3 - restriper

RAID profiles can be changed on-line, balance filters

3.4 - big metadata blocks

Support for metadata blocks larger than page size

Note

Default nodesize is 16KiB since btrfs-progs 3.12

3.4 - error handling

Generic infrastructure for graceful error handling (EIO)

3.5 - device statistics

Persistent statistics about device errors

3.5 - fsync speedup

Noticeable improvements in fsync() implementation

3.6 - qgroups

Subvolume-aware quotas

3.6 - send/receive

Ability to transfer one filesystem via a data stream (full or incremental) and apply the changes on a remote filesystem.

3.7 - extrefs

Hardlink count limit is lifted to 65536.

Note

Default since btrfs-progs 3.12

3.7 - hole punching

Implement the FALLOC_FL_PUNCH_HOLE mode of fallocate.

3.8 - device replace

Efficient replacement of existing device (add/remove in one go).

3.9 - raid 5/6 (incomplete)

Basic support for RAID5/6 profiles, no crash resiliency, replace and scrub support.

3.9 - snapshot-aware defrag

Defrag does not break links between shared extents (snapshots, reflinked files).

Note

Disabled since 3.14 (and backported to some stable kernel versions) due to problems. Has been completely removed in 5.6.

3.9 - lightweight send

A mode of send that does not add the actual file data to the stream.

3.9 - on-line label set/get

Label editable on mounted filesystems.

3.10 - skinny metadata

Reduced metadata size (format change) of extents.

Note

Default since btrfs-progs 3.18

3.10 - qgroup rescan

Sync qgroups with existing filesystem data.

3.12 - UUID tree

A map of subvolume/UUID that vastly speeds up send/receive.

3.12 - out-of-bound deduplication

Support for deduplicating extents on a given set of files.

3.14 - no-holes

No extent representation for file holes (format change), may reduce overall metadata consumption

3.14 - feature bits in sysfs

:file:`/sys/fs/btrfs` exports various bits about filesystem capabilities and feature support

3.16 - O_TMPFILE

Mode of open() to safely create a temporary file

3.16 - search ioctl v2

The extended SEARCH_TREE ioctl able to get more than a 4k data

3.18 - auto block group reclaim

Automatically remove block groups (aka. chunks) that become completely empty.

3.19 - RAID56: scrub, replace

Scrub and device replace works on RAID56 filesystems.