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Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features while also focusing on fault tolerance, repair and easy administration. Jointly developed at multiple companies, Btrfs is licensed under the GPL and open for contribution from anyone. Not too many companies have said that they are using Btrfs in production, but we welcome those who can say so on the production users page.

Contents

Stability status

For a feature status and stability please refer to the Status page. 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 Btrfs code base is under heavy development. Not only is every effort being made to ensure that it remains stable and fast but to make it more so with each and every commit. This rapid pace of development means that the filesystem improves noticeably with every new Linux release so it's highly recommended that users run the most modern kernel possible.

For benchmarks, it's recommended to test the latest stable Linux version, and not any older, as well as the latest Linux development versions. Also, it's recommended to test the various mount options such as different compression options.

As with all software, newly added features may need a few releases to stabilize.

If you find any behavior you suspect to be caused by a bug, performance issues, or have any questions about using Btrfs, please email the Btrfs mailing list (no subscription required). Please report bugs on Bugzilla.

Features

Linux has a wealth of filesystems from which to choose, but we are facing a number of challenges with scaling to the large storage subsystems that are becoming common in today's data centers. Filesystems need to scale in their ability to address and manage large storage, and also in their ability to detect, repair and tolerate errors in the data stored on disk.

Major Features Currently Implemented

  • Extent based file storage
  • 2^64 byte == 16 EiB maximum file size (practical limit is 8 EiB due to Linux VFS)
  • Space-efficient packing of small files
  • Space-efficient indexed directories
  • Dynamic inode allocation
  • Writable snapshots, read-only snapshots
  • Subvolumes (separate internal filesystem roots)
  • Checksums on data and metadata (crc32c)
  • Compression (zlib, LZO, ZSTD), heuristics
  • Integrated multiple device support
    • File Striping
    • File Mirroring
    • File Striping+Mirroring
    • Single and Dual Parity implementations (experimental, not production-ready)
  • SSD (flash storage) awareness (TRIM/Discard for reporting free blocks for reuse) and optimizations (e.g. avoiding unnecessary seek optimizations, sending writes in clusters, even if they are from unrelated files. This results in larger write operations and faster write throughput)
  • Efficient incremental backup
  • Background scrub process for finding and repairing errors of files with redundant copies
  • Online filesystem defragmentation
  • Offline filesystem check
  • In-place conversion of existing ext2/3/4 and reiserfs file systems
  • Seed devices. Create a (readonly) filesystem that acts as a template to seed other Btrfs filesystems. The original filesystem and devices are included as a readonly starting point for the new filesystem. Using copy on write, all modifications are stored on different devices; the original is unchanged.
  • Subvolume-aware quota support
  • Send/receive of subvolume changes
    • Efficient incremental filesystem mirroring
  • Batch, or out-of-band deduplication (happens after writes, not during)
  • Swapfile support

Features by kernel version

As part of the changelog you can also review

Features Currently in Development or Planned for Future Implementation

  • Online filesystem check
  • Object-level mirroring and striping
  • Alternative checksum algorithms
  • In-band deduplication (happes during writes)
  • Hot data tracking and moving to faster devices (or provided on the generic VFS layer)
  • SMR (zoned block device) support
  • DAX/persistent memory support
  • The file/directory -level encryption support (fscrypt)

News and Changelog

btrfs-progs v5.1 (May 2019)

  • check:
    • repair: flush/FUA support to avoid breaking metadata COW
    • file extents repair no longer relies on data in extent tree
    • lowmem: fix false error reports about gaps between extents
    • add inode mode check and repair for various objects
    • add check for invalid combination of nocow/compressed extents
  • device scan: new option to forget scanned devices
  • mkfs: use same chunk size as kernel for initial creation
  • dev-replace: better report when other exclusive operation is running
  • help: for syntax errors on command line, print only the relevant messages, not the whole help text
  • receive:
    • new option for quiet mode
    • on -vv print information about written ranges
    • fix endless loop with --dump on an invalid stream
  • defrag: able open files in RO mode (needs kernel support to work)
  • dump-tree: --block can be specified multiple times
  • libbtrfsutil: fix: don't close fd on error in btrfs_util_subvolume_id_fd
  • core:
    • add sync before superblock write
    • better error handling on the transaction commit path
    • try to find best copy when reading tree blocks
    • update backup roots on commit transaction
  • other:
    • fuzz tests pass and are enabled in CI
    • cleanups
    • new tests

linux v5.1 (May 2019)

  • new features, hilights
    • zstd compression levels can be set as mount options
    • new ioctl to unregister scanned devices
    • scrub prints messages about start/stop/cancel to the log
  • other changes
    • qgroups skip some work (est. speedup during balance 20%)
    • reclaim vs GFP_KERNEL fixes
    • fsync fixes for rename/unlink/rmdir
    • improved enospc handling on a highly fragmented filesystem
    • no trim on filesystem with unreplayed log

GRUB supports ZSTD

  • support for booting from filesystems with files compressed by ZSTD has been merged to GRUB2 [1] (11/2018)

GRUB supports RAID 5/6 boot

  • support for booting from filesystems with RAID5/6 profiles has been merged to GRUB2 [2] (10/2018)

Read about past releases in the Changelog

Documentation

Guides and usage information

External Btrfs Documentation / Guides

Links to Btrfs documentation of various Linux distributions:

Project information/Contact

Manual pages

  • Original wiki documentation (obsolete, will be removed)

Developer documentation

  • Developer's FAQ — hints and answers for contributors and developers, general information about patch formatting
  • Development notes — notes, hints, checklists for specific implementation tasks (eg. adding new ioctls)
  • Code documentation — trees, source files, sample code for manipulating trees
  • Data Structures — detailed on-disk data structures
  • Trees — detailed in-tree representation of files and directories
  • Original COW B-tree: Source code in C that implements the COW B-tree algorithms repository. Written by Ohad Rodeh at IBM Research in 2006, and released under a BSD license. This is a reference implementation, that works in user space.
  • Unmerged features
    • In-band (write) time deduplication

Source code download

Wiki editing

The wiki contributions are welcome! Please create an account and wait for approval (this is a necessary spam protection). You can try to catch some of the wiki admins on IRC and expedite the account creation.

Articles, presentations, podcasts


Historical resources

Links to old or obsolete documentation, articles. Kept for historical reasons. Stuff that's more than 3 years old.

Articles, presentations, podcasts

Benchmarks

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