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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 (stable)
------------
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Allow user override of qgroup accounting and make it temporarily out
of date e.g. in case when there are several subvolumes deleted and the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).
6.6 (stable)
------------
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 (stable)
-------------
-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 - tree-checker update for inline extent references
Update tree-checker to detect more wrong inline extent references.
6.14 - support READ_VERITY_METADATA ioctl
Add support for FS_IOC_READ_VERITY_METADATA to directly query the
Merkle tree, descriptor and signature blocks for fs-verity enabled
files.
6.14 - *experimental* read balancing policies
Add more read balancing policies, configurable in :file:`/sys/fs/btrfs/FSID/read_policy`
or as module parameter *read_policy*. Newly added *round-robin*,
*devid:N* (select a specific mirror number *N*).
6.14 - encoded write integration with io_uring
The io_uring subsystem understands a command that is directed to
Btrfs encoded write ioctl.
6.15 - always do buffered IO for files with checksums
Direct IO may lead to data and their checksums mismatch. Use the direct
to buffered fallback in case the file has checksums. This has a negative
performance impact.
6.15 - negative (fast) levels for zstd
Mount options for *zstd* compression accept negative values *-1..-15*
match the levels. They provide faster compression at the cost of worse
ratio.
6.15 - *(debug builds)* accept 2K block size on x86_64
For testing the *subpage* block size feature, the size of 2K is accepted
on x86_64 which has 4K pages.
6.15 - defrag ioctl accepts negative zstd levels
The defrag ioctl also accepts the negative zstd levels that can be set as
mount option.
6.17 (latest)
-------------
6.17 - track current commit duration in commit_stats
Add entry to :file:`commit_stats` to detect commit stalls, for
debugging or monitoring purposes.
6.17 - *experimental* large folio support
Large folios abstract contiguous page ranges representing some filesystem
data or metadata as one structure instead of several ones. This simplifies
code and has a positive impact on performance. As it touches the core
data structure it is not enabled by default.
6.17 - restrict writes to mounted devices
Any btrfs mounted device cannot be opened for writes.
6.17 - defrag ioctl can force no compression
The defrag ioctl was not able to uncompress a given range, now it's
possible.
6.17 - send (v2 protocol) uses fallocate for hole punching
File holes, ranges not representing data, were emulated by a zero
filled data. This is less efficient than puching holes.
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.
5.4 (stable)
------------
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 (stable)
-------------
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 (stable)
-------------
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.
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