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= Overview =
= Overview =
Revision as of 12:23, 5 June 2017
For a list of features by their introduction, please see the table Changelog#By_feature.
The table below aims to serve as an overview for the stability status of the features BTRFS supports. While a feature may be functionally safe and reliable, it does not necessarily mean that its useful, for example in meeting your performance expectations for your specific workload.
The current table is based on kernel 4.9 LTS
|Trim (aka. discard)||OK||fstrim and mounted with -o discard (has performance implications)|
|Defrag||mostly OK||extents get unshared (see below)|
|Compression & deduplication|
|Compression||mostly OK||(needs verification and source) auto-repair and compression may crash|
|Out-of-band dedupe||mostly OK||performance issues|
|File range cloning||mostly OK||(reflink), heavily referenced extents have a noticeable performance hit|
|Auto-repair||OK||automatically repair from a correct spare copy if possible (dup, raid1, raid10)|
|Scrub + RAID56||Unstable||will verify but not repair|
|nodatacow||OK||Nodatacow does not checksum data, see Manpage/btrfs(5).|
|Device replace||mostly OK||gets stuck on devices with bad sectors|
|Block group profile|
|Single (block group profile)||OK|
|DUP (block group profile)||OK|
|RAID1||mostly OK||Needs at least two available devices always. Can get stuck in irreversible read-only mode if only one device is present.|
|RAID10||mostly OK||Needs to be able to create two copies always. Can get stuck in irreversible read-only mode if only one copy can be made.|
|RAID56||Unstable||write hole still exists, parity not checksummed|
|Mixed block groups||OK||see documentation|
|Filesystem resize||OK||shrink, grow|
|Offline UUID change||OK|
|Send||OK||corner cases may still exist|
|Seeding||OK||should be better documented|
|Quotas, qgroups||mostly OK|
|Free space tree||mostly OK||see below|
|no-holes||OK||see documentation for compatibility|
|skinny-metadata||OK||see documentation for compatibility|
|extended-refs||OK||see documentation for compatibility|
- OK: should be safe to use, no known defficiencies
- mostly OK: safe for general use, there are some known problems
- Unstable: do not use for other then testing purposes, known severe problems, missing implementation of some core part
Note to editors: please update the table if:
- you're sure you know the answer, eg. a bug number for a bug that lowers the feature status
- a particular feature combination that has a different status and is worth mentioning separately
- there's a missing entry (put TBD to the status)
- a reference could be enhanced by an actual link to documentation (wiki, manual pages)
The page edits are watched by wiki admins, do not worry to edit.
Details that do not fit the table
The data affected by the defragmentation process will be newly written and will consume new space, the links to the original extents will not be kept. See also Manpage/btrfs-filesystem. Though autodefrag affects newly written data, it can read a few adjacent blocks (up to 64k) and write the contiguous extent to a new location. The adjacent blocks will be unshared. This happens on a smaller scale than the on-demand defrag and doesn't have the same impact.
Free space tree
- btrfs-progs support is read-only, ie. fsck can check the filesystem but is not able to keep the FST consistent and thus cannot run in repair mode
- the free space tree can be cleared using 'btrfs check --clear-space-cache v2' and will be rebuilt at next mount
Compatibility and historical references:
- btrfs-progs versions before v4.7.3 might accidentally do writes to the filesystem, but since there's no way to invalidate the FST, this causes inconsistency and possible corruption (using a piece of space twice). If you have made changes (btrfstune, repair, ...) to a FST enabled filesystem with btrfs progs, then mount with clear_cache,space_cache=v2 and hope the space written to was not reused yet. (see Status of free-space-tree feature)
- (fixed in linux 4.9) runtime support: fine on little-endian machines (x86*), known to be broken on big-endian (sparc64), see sparc64: btrfs module fails to load on big-endian machines
The filesystem disk format is stable. This means it is not expected to change unless there are very strong reasons to do so. If there is a format change, filesystems which implement the previous disk format will continue to be mountable and usable by newer kernels.
The core of the on-disk format that comprises building blocks of the filesystem:
- layout of the main data structures, eg. superblock, b-tree nodes, b-tree keys, block headers
- the COW mechanism, based on the original design of Ohad Rodeh's paper "Shadowing and clones"
Newly introduced features build on top of the above and could add specific structures. If a backward compatibility is not possible to maintain, a bit in the filesystem superblock denotes that and the level of incompatibility (full, read-only mount possible).