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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
    • 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)
  • 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
  • Seeding 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 and backup
  • 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)
  • fsverity integration

Features by kernel version

Features Currently in Development or Planned for Future Implementation

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

Documentation

Documentation

Guides and usage information

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

btrfs-progs v5.17 (Apr 2022)

  • check:
    • repair wrong num_devices in superblock
    • recognize overly long xattr names
    • fix wrong total bytes check for seed device
  • auto-repair on read on RAID56
  • property set: unify handling of empty value to mean default, changed meaning for property 'compression' to allow reset to default and to set NOCOMPRESS, since kernel 5.14
  • fixes:
    • dump-tree: print fs-verity items
    • fix location of system chunk on zoned filesystem
    • do not allow setting seeding flag on a filesystem with dirty log
    • mkfs and subpage support: use sectorsize as nodesize fallback for mixed profiles
  • preparatory work for extent tree v2, global roots
  • experimental feature (unstable interface, not built by default, do not use for production): btrfstune: option --csum to switch checksum algorithm
  • other:
    • update documentation build, remove asciidocs leftovers
    • update fssum to consider xattrs

util-linux v2.38 (Apr 2022)

blk* utilities and libraries finally recognize btrfs formatted with zoned mode

linux v5.17 (Mar 2022)

Features:

  • make send work with concurrent block group relocation
  • new exclusive operation 'balance paused' to allow adding a device to filesystem with paused balance
  • new sysfs file for fsid stored in the per-device directory to help distinguish devices when seeding is enabled

Performance:

  • less metadata needed for directory logging, directory deletion is 20-40% faster
  • in zoned mode, cache zone information during mount to speed up repeated queries (about 50% speedup)
  • free space tree entries get indexed and searched by size (latency -30%, search run time -30%)
  • less contention in tree node locking when inserting a key and no splits are needed (files/sec in fsmark improves by 1-20%)

Fixes:

  • defrag rewrite from 5.16 fixed
  • get rid of warning when mounted with flushoncommit

Core:

  • global reserve stealing got simplified and cleaned up in evict
  • more preparatory work for extent tree v2
  • remove readahead framework
  • error handling improvements

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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