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btrfs-progs/Documentation/Feature-by-version.rst

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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 [[Status]] page.
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 16k 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 64k
.. 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. Will be enabled in the future.
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
/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.
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 endianity 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 (/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
<i>ref-verify</i>, 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.
[https://git.kernel.org/linus/d24a67b2d997c860a42516076f3315c2ad2d2884
Read more.]
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 [http://man7.org/linux/man-pages/man2/fallocate.2.html
*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.
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 */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=
Additional modes for mount option *rescue=*: ignorebadroots/ibadroots,
ignoredatacsums/idatacsums. All are exported in sysfs.
5.12 - zoned mode
Support for zoned devices with special allocation/write mode to
fixed-size zones. See [[Zoned]].
5.13 - supported_sectorsizes in sysfs
List supported sector sizes in sysfs file /sys/fs/btrfs/features/supported_sectorsizes
5.14 - sysfs scrub bw limit
Tunable bandwidth limit
(/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 /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_V2m 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
/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.
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 recompress (or reencrypt) on the receiving side.
6.0 - sysfs exports commit stats
The 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
/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
/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 '/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
/sys/sys/fs/btrfs/FSID/qgroups/ .
6.1 - check that subperblock 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 '/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
'/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 syfs and now
available via DEV_INFO ioctl.
6.3 - send utimes cache, reduced stream size
Utimes for directories are emitted into the send steram only when
finalizing the directory, the cache also gains significant speedups (up
to 10x).
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