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Btrfs is a new copy on write (CoW) filesystem for Linux aimed at implementing advanced features while 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

The filesystem disk format is no longer unstable, and it's not expected to change unless there are strong reasons to do so. If there is a format change, file systems with a unchanged format will continue to be mountable and usable by newer kernels.

The Btrfs code base is under heavy development. Every effort is being made to keep it stable and fast. Due to the fast development speed, the state of development of the filesystem improves noticeably with every new Linux version, so it's recommended to run the most modern kernel possible.

For benchmarks, it's recommended to test the latest stable Linux version, and not any older. If possible, it's also recommendable to test the latest Linux development version. Also, it's recommended to test the different options, f.e. different compression options.

Newly added features may need a few releases to stabilize.

If you have any bug, problems, performance issues or questions while using Btrfs, please email the Btrfs mailing list (no subscription required). Please report bugs also 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.

The main Btrfs features available at the moment include:

  • 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 and LZO)
  • Integrated multiple device support
    • File Striping, File Mirroring, File Striping+Mirroring, Striping with Single and Dual Parity implementations
  • 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 fixing errors on files with redundant copies
  • Online filesystem defragmentation
  • Offline filesystem check
  • In-place conversion of existing ext3/4 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)

Additional features in development, or planned, include:

  • Fast offline filesystem check
  • Online filesystem check
  • Object-level mirroring and striping
  • Alternative checksum algorithms
  • In-band deduplication (happens during writes)
  • Other compression methods (snappy, LZ4)
  • Hot data tracking and moving to faster devices (currently being pushed as a generic feature available through VFS)

News

btrfs-progs 4.5.2 (Mar 2016)

  • new/moved command: btrfs-calc-stats -> btrfs inspect tree-stats
  • check: fix false alert for metadata blocks crossing stripe boundary
  • check: catch when qgroup numbers mismatch
  • check: detect running quota rescan and report mismatches
  • balance start: add safety delay before doing a full balance
  • fi sync: is now silent
  • fi show: don't miss filesystems with partially matching uuids
  • dev ready: accept only one argument for device
  • dev stats: print "devid:N" for a missing device instead of "(null)"
  • other:
    • lowest supported version of e2fsprogs is 1.41
    • minor cleanups, test updates

Linux v4.5 (Mar 2016)

  • free space cache v2: an incompat feature to track the free space cache as a b-tree
  • balance:
    • '-dconvert=dup' supported
    • continue but warn if metadata have lower redundancy than data
  • fixes:
    • trim does not overwrite bootloader area (introduced in 4.3, fixed in 4.4.x stable kernels)
  • assorted bugfixes, improvements or cleanups

In coreutils 8.24

  • mv will attempt reflink before falling back to standard copy [1] (release notes)

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

Using the built-in tools

  • Original wiki documentation (obsolete, will be removed)

Developer documentation

  • Developer's FAQ — hints and answers for contributors and developers
  • 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.

Source code download

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

Personal tools