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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

Features by kernel version

As part of the changelog you can also review

• features by kernel version

Features Currently in Development or Planned for Future Implementation

• SMR (zoned block device) support
• DAX/persistent memory support
• The file/directory -level encryption support (fscrypt)

Documentation

Guides and usage information

• Getting started — first steps, distributions with btrfs support
  • Mount options
• FAQ — About the btrfs project and filesystem
• UseCases — Recipes for how to do stuff with btrfs
• SysadminGuide — A more in-depth guide to btrfs's concepts and a bit of its internals, to answer all those "but what is a subvolume?" kind of questions.
• Multiple devices – A guide to the RAID features of Btrfs
• Conversion from Ext3 and Ext4 or reiserfs
• Problem FAQ — Commonly-encountered problems and solutions.
  • Gotchas — lists known bugs and issues, but not necessarily solutions.

External Btrfs Documentation / Guides

Links to Btrfs documentation of various Linux distributions:

• "The Btrfs File System" chapter in the Oracle Linux 6 Administrator's Solutions Guide
• Oracle Linux Hands-on lab - Storage Management with Btrfs
• Major File Systems in Linux chapter in the SLES 15 Storage Administration Guide
• Btrfs chapter in the Red Hat Enterprise Linux 6 Storage Administration Guide
• Btrfs Wiki page on the Ubuntu Community Help Wiki
• Btrfs Wiki page on the Arch Linux Wiki
• BTRFS Fun Wiki page on the Funtoo Wiki
• Marc MERLIN's Btrfs talk at Linuxcon JP 2014 which gives an overview of Btrfs, best practices, and its more interesting features.

Manual pages

• Manual pages generated from git (complete list):
  • btrfs — main administration tool
  • mkfs.btrfs — creating the filesystem
  • btrfs check — repairing file systems
  • btrfs-convert — tool to convert in-place from ext2/3/4 filesystems to btrfs. For a greater detail of how the algorithm works, please see the Conversion from Ext3 page.

• Original wiki documentation (obsolete, will be removed)
  • restore and find-root — utilities to find and restore data from an unmountable filesystem

Developer documentation

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

• Developer documentation on Github - collection of documents describing internals of BTRFS
• 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
• On-disk Format - Details of data structures written on disk
• Trees — detailed in-tree representation of files and directories

• Btrfs design — design notes (possibly out of date in places)
• Multiple Device Support — design notes
• ENOSPC — Current ENOSPC design issues
• Design_notes_on_Send/Receive — notes from initial impelentation, protocol V2 updates draft
• Qgroups status quo - notes on some qgroups observations/pain points

• Debugging Btrfs with GDB
• Writing patch for btrfs
• Btree_Items - Mapping from Btrfs key to item-data
• Resolving_Extent_Backrefs - How back references are resolved to root owners

• 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
    • User notes on dedupe — User/tester notes for using in-band deduplication feature

News

btrfs-progs v5.11.1 (Mar 2021)

• properly format checksums when a mismatch is reported
• check: fix false alert on tree block crossing 64K page boundary
• convert:
  • refuse to convert filesystem with 'needs_recovery'
  • update documentation to require fsck before conversion
• balance convert: fix raid56 warning when converting other profiles
• fi resize: improved summary

btrfs-progs v5.11 (Mar 2021)

• fix device path canonicalization for device mapper devices
• receive: remove workaround for setting capabilities, all stable kernels have been patched
• receive: fix duplicate mount path detection
• rescue: new subcommand create-control-device
• device stats: minor fix for plain text format output
• build: detect if e2fsprogs support 64bit timestamps
• build: drop libmount, required functionality has been reimplemented
• mkfs: warn when raid56 is used
• balance convert: warn when raid56 is used
• other
  • new and updated tests
  • documentation updates: seeding device, raid56 status
  • CI updates: docker images for various distros

linux v5.11 (Feb 2020)

• new mount option rescue, various modes how to access a damaged filesystem
• sysfs updates: filesystem generation, supported rescue modes, read mirror policy
• removed feature: mount -o inode_cache
• free space tree fixes, v1 cache removed during conversion
• core
  • locking switched to standard rw semaphores
  • direct IO ported to iomap infrastructure
  • zoned allocation mode preparation
  • subpage blocksize preparation
  • various performance improvements (skipping unnecessary work)

Changelog

Read about past releases in the separate Changelog page

• features by kernel version
• kernel changes for each release
• btrfs-progs changes for each release

Source code download

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

• repository for kernel pull requests: git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git (gitweb)
• kernel development repository: git://github.com/kdave/btrfs-devel.git (web access)
• kernel development repository (mirror): git://gitlab.com/kdave/btrfs-devel.git (web access)
• btrfs-progs development and release repository: git://github.com/kdave/btrfs-progs.git (web access)
• released tarballs of btrfs-progs: https://mirrors.edge.kernel.org/pub/linux/kernel/people/kdave/btrfs-progs/

Articles, presentations, podcasts

• Video: Deploying Btrfs at Facebook Scale by Josef Bacik at the Open Source Summit 2020 (2020-06-29)
• Video: btrfs is awesome, except when it isn't by Richard Brown at openSUSE Conferece 2018 (2018-05-25)
• Video: btrfs: The Best Filesystem You've Never Heard Of by poiupoiu at PhreakNIC 21 (2017-11-3)
• Video TUT91782 Getting the most out of the btrfs filesystem by Thorsthen Kukuk and Jeff Mahoney (SUSECON, 2017)
• Video: NYLUG Presents: Chris Mason on Btrfs (May 14th 2015) by Chris Mason at the 192nd meeting of the NYLUG
• Video: Why you should consider using btrfs ... like Google does. by Marc Merlin at linux.conf.au 2015. talk slides
• Article: Bitrot and atomic COWs: Inside “next-gen” filesystems (ars technica, 2014/01)
• Article: Btrfs: Subvolumes and snapshots (LWN.net, 2014/01)
• Article: Btrfs: Working with multiple devices (LWN.net, 2013/12)
• Article: Btrfs: Getting started (LWN.net, 2013/12)
• Article: Btrfs hands on: An extremely cool file system (ZDNet, 2013/11)
• Technical report: Visualizating Block IO Workloads. Section six shows a visual comparison of the IO patterns for BTRFS, XFS, and EXT4. Submitted to ACM Transactions on Storage, November 2013.
• Paper: BTRFS: The Linux B-Tree Filesystem describing the overall concepts and architecture, appeared in ACM Transactions on Storage, August 2013. Includes a detailed comparison with ZFS. There is a free ACM authorized link, from O. Rodeh's [1] page. Otherwise, try IBM Research link

Historical resources

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

Articles, presentations, podcasts

• Article: A Beginner's Guide To Btrfs (2012-11-26)
• Article: How I Use the Advanced Capabilities of Btrfs by Margaret Bierman with Lenz Grimmer (2012-08-11)
• Article: How I Got Started with the Btrfs File System for Oracle Linux by Margaret Bierman with Lenz Grimmer (2012-07-11)
• Video: Btrfs Filesystem: Status and New Features by Chris Mason at the 2012 Linux Foundation Collaboration Summit (2012-04-05)
• Article Btrfs - The Swiss Army Knife Of Storage by Josef Bacik (2012-02)
• Video: I Can't Believe This is Butter! A tour of btrfs by Avi Miller at linux.conf.au (2012-01-18)
• Article: A btrfs update at LinuxCon Europe on lwn.net (2011-11-02)
• Podcast: Podcast with Chris Mason: Btrfs overview and status update (2011-07-13)
• Video: The Story So Far by Josef Bacik at LinuxCon Brazil (2010-08-31)
• Article: Linux.com Weekend Project: Get Started with Btrfs (2010-10-15)
• Webcast: State of "Btrfs" File System for Linux by Chris Mason (2010-08-26) [email registration and flash required]
• Article: Valerie Aurora: A short history of btrfs (2009-07-22)

Project information/Contact

• Changelog — history of changes in linux kernel wrt btrfs
  • features added by release
• Development statistics — contributors, commits, lines
• Glossary
• Contact information:
  • Btrfs mailing list
  • IRC on freenode.net in the channel #btrfs
  • Reporting bugs:
    • for kernel code see the Bugzilla FAQ, quick tip: use product File System and component btrfs.
    • for btrfs-progs it's either bugzilla or github issues
    • for read-only documentation exported on wiki eg. manual pages as github issues
• Project ideas
• Cleanup ideas
• Userspace tools projects

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.