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btrfs

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. Its main features and benefits are:

  • Snapshots which do not make the full copy of files
  • RAID - support for software-based RAID 0, RAID 1, RAID 10
  • Self-healing - checksums for data and metadata, automatic detection of silent data corruptions

Development of Btrfs started in 2007. Since that time, Btrfs is a part of the Linux kernel and is under active development.

Jointly developed at multiple companies, Btrfs is licensed under the GPL and open for contribution from anyone.

List of companies using btrfs in production.

Development and Issue Reporting

For feature status, please refer to the Status page.

The Btrfs code base is stable. However, new features are still under development. Every effort is made to ensure that it remains stable and fast at 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.

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, xxhash, sha256, blake2b)
  • 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
  • Tree-checker, post-read and pre-write metadata verification
  • Zoned mode support (SMR/ZBC/ZNS friendly allocation)

Features by kernel version

As part of the changelog you can also review

Features Currently in Development or Planned for Future Implementation

  • DAX/persistent memory support
  • The file/directory -level encryption support (fscrypt)
  • fsverity integration

Documentation

Guides and usage information

External Btrfs Documentation / Guides

Links to Btrfs documentation of various Linux distributions:

Manual pages

  • Original wiki documentation (obsolete, will be removed)

Developer documentation

  • Development setup - how to build btrfs from sources and prepare a development environment
  • 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

News

IRC channel at libera.chat

The #btrfs channel is at libera.chat, matrix.org bridge works (persistent room #btrfs:matrix.org).

btrfs-progs v5.13.1 (Jul 2021)

  • build: fix build on musl libc due to missing definition of NAME_MAX
  • check:
    • batch more work into one transaction when clearing v1 free space inodes
    • detect directoris with wrong number of links
  • libbtrfsutil: fix race between subvolume iterator and deletion
  • mkfs: be more specific about supported profiles for zoned device
  • documentation updates

linux v5.13 (Jun 2021)

  • User visible improvements
    • readahead for send, improving run time of full send by 10% and for incremental by 25%
    • make reflinks respect O_SYNC, O_DSYNC and S_SYNC flags
    • export supported sectorsize values in sysfs (currently only page size, more once full subpage support lands)
    • more graceful errors and warnings on 32bit systems when logical addresses for metadata reach the limit posed by unsigned long in page::index
      • error: fail mount if there's a metadata block beyond the limit
      • error: new metadata block would be at unreachable address
      • warn when 5/8th of the limit is reached, for 4K page systems it's 10T, for 64K page it's 160T
    • zoned mode
      • relocated zones get reset at the end instead of discard
      • automatic background reclaim of zones that have 75%+ of unusable space, the threshold is tunable in sysfs
  • Fixes
    • fix inefficient preemptive reclaim calculations
    • fix exhaustion of the system chunk array due to concurrent allocations
    • fix fallback to no compression when racing with remount
    • fix unmountable seed device after fstrim
    • fix fiemap to print extents that could get misreported due to internal extent splitting and logical merging for fiemap output
    • preemptive fix for dm-crypt on zoned device that does not properly advertise zoned support
  • Core changes
    • add inode lock to synchronize mmap and other block updates (eg. deduplication, fallocate, fsync)
    • subpage support (continued)
      • metadata changes now support read and write
    • error handling through out relocation call paths
    • many other cleanups and code simplifications

Source code download

Btrfs source repositories describes purpose and contents, here are a few quick links:

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

Project information/Contact

Wiki accounts, editing

The wiki contributions are welcome! Please create an account and wait for approval (this is a necessary spam protection and we cannot remove it). You can try to catch some of the wiki admins on IRC (or ping user 'kdave' in a query) to expedite the account creation.

The registration requires full name for account but it's not mandatory from our perspective. The wiki User and User talk pages are created automatically but removed after account is approved. If you want to use the pages, create them manually, they won't be deleted.

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