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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, 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 for extra robustness

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
  • In-band deduplication (happens 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

linux v5.6 (Mar 2020)

  • Highlights:
    • async discard
      • "mount -o discard=async" to enable it
      • freed extents are not discarded immediatelly, but grouped together and trimmed later, with IO rate limiting
      • the actual discard IO requests have been moved out of transaction commit to a worker thread, improving commit latency
      • IO rate and request size can be tuned by sysfs files, for now enabled only with CONFIG_BTRFS_DEBUG as we might need to add/delete the files and don't have a stable-ish ABI for general use, defaults are conservative
    • export device state info in sysfs, eg. missing, writeable
    • no discard of extents known to be untouched on disk (eg. after reservation)
    • device stats reset is logged with process name and PID that called the ioctl
  • Core changes:
    • qgroup assign returns ENOTCONN when quotas not enabled, used to return EINVAL that was confusing
    • device closing does not need to allocate memory anymore
    • snapshot aware code got removed, disabled for years due to performance problems, reimplmentation will allow to select wheter defrag breaks or does not break COW on shared extents
    • tree-checker:
      • check leaf chunk item size, cross check against number of stripes
      • verify location keys for DIR_ITEM, DIR_INDEX and XATTR items
      • new self test for physical -> logical mapping code, used for super block range exclusion
  • Fixes:
    • fix missing hole after hole punching and fsync when using NO_HOLES
    • writeback: range cyclic mode could miss some dirty pages and lead to OOM
    • two more corner cases for metadata_uuid change after power loss during the change
    • fix infinite loop during fsync after mix of rename operations
  • see pull request

Notable fixes (Feb 2020)

  • fix for df (statfs) reporting 0 after heavy metadata operation like balance
    • 5.5.2, 5.4.18

btrfs-progs v5.4.1 (Jan 2020)

  • build: fix docbook5 build
  • check: do extra verification of extent items, inode items and chunks
  • qgroup: return ENOTCONN if quotas not running (needs updated kernel)
  • other: various test fixups

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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