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Developer(s) Matthew Dillon
Full name HAMMER
Introduced July 21, 2008 with DragonFly BSD 2.0[1]
Directory contents B+ tree[2]
Max. volume size 1 EiB[2]
Forks No
File system permissions UNIX permissions
Transparent compression Yes[3]
Data deduplication On demand
Supported operating systems DragonFly BSD

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.[4] HAMMER also supports data block deduplication, meaning that identical data blocks will be stored only once on a file system.[5]


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.[4][6][7][8]

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.[6][9][10]


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.[6][8]


The following performance-related improvements were introduced on July 2011:[11]

  • 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 then increasingly popular ZFS.

As of June 2014, the HAMMER2 file system was actively developed by Matthew Dillon, who initially planned to bring it up to minimal working state by July 2012 and ship the final version in 2013.[12] During Google Summer of Code 2013 Daniel Flores implemented compression in HAMMER2 using LZ4 and zlib algorithms.[13] On June 4, 2014, DragonFly 3.8.0 was released featuring support for HAMMER2, although the file system was said to be not ready for use.[14]

See also[edit]


  1. ^ Larabel, Michael (2011-01-07). "Can DragonFlyBSD's HAMMER Compete With Btrfs, ZFS?". Phoronix. Retrieved 2014-05-26. 
  2. ^ a b Dillon, Matthew (2008-06-21). "THE HAMMER FILESYSTEM" (PDF). Retrieved 2009-03-02. 
  3. ^ https://www.phoronix.com/scan.php?page=news_item&px=HAMMER2-LZ4-Default
  4. ^ a b hammer(5) – DragonFly BSD File Formats Manual
  5. ^ Sherrill, Justin (2010-11-07). "Deduplication arrives". Retrieved 2011-11-28. 
  6. ^ a b c "HAMMER". DragonFly BSD. Retrieved 2011-11-28. 
  7. ^ Vervloesem, Koen (2010-04-21). "DragonFly BSD 2.6: towards a free clustering operating system". LWN.net. Retrieved 2014-05-26. 
  8. ^ a b 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 2014-05-27. Retrieved 2014-05-25. 
  9. ^ hammer(8) – DragonFly BSD System Maintenance and Operation Commands Manual
  10. ^ Kemp, Juliet (2008-08-04). "Tip of the Trade: Dragonfly BSD and the Hammer Filesystem". ServerWatch. Retrieved 2014-05-26. 
  11. ^ Dillon, Matthew (2011-07-22). "git: HAMMER VFS - Add code to reduce frontend vs flusher locking conflicts". commits (Mailing list). 
  12. ^ Dillon, Matthew (2012-02-08). "DESIGN document for HAMMER2 (08-Feb-2012 update)". users (Mailing list). 
  13. ^ "Block compression feature in HAMMER2". GSoC 2013. Google. Retrieved 2014-06-05. 
  14. ^ "DragonFly Release 3.8". DragonFly BSD. 2014-06-04. Retrieved 2014-06-05. 

External links[edit]