HAMMER (file system)

HAMMER is a high-availability 64-bit file system developed by Matthew Dillon for DragonFly BSD using B+ trees. Its major features include infinite NFS-exportable snapshots, master-multislave operation, configurable history retention, fsckless-mount, and checksums to deal with data corruption.[5] HAMMER also supports data block deduplication, meaning that identical data blocks will be stored only once on a file system.[6] A successor, HAMMER2, was announced in 2011 and became the default in Dragonfly 5.2 (April 2018).[7]

Developer(s)Matthew Dillon
Full nameHAMMER
Introduced21 July 2008 with DragonFly BSD 2.0[1][2]
Directory contentsModified B+ tree[3]
Max. volume size1 EiB[3]
File system permissionsUNIX permissions
Transparent compressionYes[4]
Data deduplicationOn demand
Supported operating systemsDragonFly BSD


HAMMER file system provides configurable fine-grained and coarse-grained filesystem histories with online snapshots availability. Up to 65536 master (read-write) and slave (read-only) pseudo file systems (PFSs), with independent individual retention parameters and inode numbering, may be created for each file system; PFS may be mirrored to multiple slaves both locally or over network connection with near real-time performance. No file system checking is required on remount.[5][8][9][10]

HAMMER supports volumes up to 1 EiB of storage capacity. File system supports CRC checksumming of data and metadata, online layout correction and data deduplication, and dynamic inodes allocation with effectively unlimited number of inodes.[8][11][12]


Currently, regular maintenance is required to keep the file system clean and regain space after file deletions. By default, a cron job performs the necessary actions on DragonFly BSD daily. HAMMER does not support multi-master configurations.[8][10]


HAMMER is optimised to reduce the number of physical IO operations to cover the most likely path,[13] ensuring sequential access for optimal performance.

The following performance-related improvements were introduced on July 2011:[14]

  • Increased disk read speed in certain scenarios by implementing pulse-width modulated time-domain multiplexer on B-tree cursor operation
  • Removed a deadlock stalling issue
  • Improved read performance during heavy, concurrent file write operations


HAMMER was developed specifically for DragonFly BSD to provide a feature-rich yet better designed analogue of the then increasingly popular ZFS.

HAMMER was declared production-ready with DragonFly 2.2 in 2009;[9] in 2012, design-level work has shifted onto HAMMER2, which was declared stable with DragonFly 5.2 in 2018.

As of 2019, HAMMER is now often referred to as HAMMER1 to avoid confusion with HAMMER2, although an official renaming hasn't happened. Both filesystems are independent of each other due to different on-disk formats,[15][16] and continue to receive separate updates and improvements independently.[17]

See also


  1. "В состав DragonFlyBSD 2.0 будет включена файловая система HAMMER". Linux.org.ru (in Russian). 2007-10-14. Retrieved 2019-08-21.
  2. Larabel, Michael (7 January 2011). "Can DragonFlyBSD's HAMMER Compete With Btrfs, ZFS?". Phoronix. Retrieved 2014-05-26.
  3. Dillon, Matthew (21 June 2008). "THE HAMMER FILESYSTEM" (PDF). Retrieved 2009-03-02.
  4. https://www.phoronix.com/scan.php?page=news_item&px=HAMMER2-LZ4-Default
  5. hammer(5)  DragonFly BSD File Formats Manual
  6. Sherrill, Justin (7 November 2010). "Deduplication arrives". Retrieved 2011-11-28.
  7. Dillon, Matthew (11 May 2011). "HAMMER2 announcement". users (Mailing list).
  8. "HAMMER". DragonFly BSD. Retrieved 2011-11-28.
  9. Vervloesem, Koen (21 April 2010). "DragonFly BSD 2.6: towards a free clustering operating system". LWN.net. Retrieved 2014-05-26.
  10. George, Siju (May 2014). "Working with Hammer File System and PFSes" (PDF). BSD Magazine. Warsaw, Poland: Hakin9 Media SK. 8 (5): 18–23. Archived from the original (PDF) on 27 May 2014. Retrieved 2014-05-25.
  11. hammer(8)  DragonFly BSD System Maintenance and Operation Commands Manual
  12. Kemp, Juliet (4 August 2008). "Tip of the Trade: Dragonfly BSD and the Hammer Filesystem". ServerWatch. Retrieved 2014-05-26.
  13. Jeremy Andrews (2007-10-14). "HAMMER Performance". KernelTrap. Retrieved 2019-08-21.
  14. Dillon, Matthew (22 July 2011). "git: HAMMER VFS - Add code to reduce frontend vs flusher locking conflicts". commits (Mailing list).
  15. Matthew Dillon (2017-09-23). "hammer_disk.h". BSD Cross Reference. DragonFly BSD. Retrieved 2019-03-06.
  16. Matthew Dillon (2018-05-05). "hammer2_disk.h". BSD Cross Reference. DragonFly BSD. Retrieved 2019-03-06.
  17. Matthew Dillon (2017-03-27). "git: hammer - HAMMER Version 7". commits@DragonFly (Mailing list). Retrieved 2019-03-06.
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