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| Developer(s) | Matthew Dillon |
|---|---|
| Full name | HAMMER |
| Introduced | July 21, 2008; 17 years ago (2008-07-21) withDragonFly BSD 2.0[1][2] |
| Structures | |
| Directory contents | ModifiedB+ tree[3] |
| Limits | |
| Max volume size | 1EiB[3] |
| Features | |
| Forks | No |
| File system permissions | UNIX permissions |
| Transparent compression | Yes[4] |
| Data deduplication | On demand |
| Other | |
| Supported operating systems | DragonFly BSD |
HAMMER is a high-availability64-bitfile system developed byMatthew Dillon forDragonFly BSD usingB+ trees. Its major features include infinite NFS-exportablesnapshots,master–multislave operation, configurable history retention,fsckless-mount, andchecksums to deal withdata corruption.[5] HAMMER also supports data blockdeduplication, 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]
HAMMER file system provides configurable fine-grained and coarse-grained filesystem histories with online snapshots availability. Up to 65536master (read–write) andslave (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 onremount.[5][8][9][10]
HAMMER supports volumes up to 1 EiB of storage capacity. File system supportsCRC checksumming of data and metadata, online layout correction anddata deduplication, and dynamicinodes allocation with an effectively unlimited number of inodes.[8][11][12]
As of May 2020[update], regular maintenance is required to keep the file system clean and regain space after file deletions. By default, acron job performs the necessary actions on DragonFly BSD daily. HAMMER does not support multi-master configurations.[8][10]
HAMMER is optimized to reduce the number of physical I/O operations to cover the most likely path,[13] ensuringsequential access for optimal performance.
The following performance-related improvements were introduced inJuly 2011:[14]
HAMMER was developed specifically for DragonFly BSD to provide a feature-rich yet better designed analogue[according to whom?] of the then increasingly popularZFS.
HAMMER was declared production-ready with DragonFly 2.2 in 2009;[9] in 2012, design-level work shifted ontoHAMMER2, which was declared stable with DragonFly 5.2 in 2018.
As of 2019[update], HAMMER is now often referred to as HAMMER1 to avoid confusion with HAMMER2, although an official renaming has not 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]
hammer(5) – DragonFly BSD File FormatsManualhammer(8) – DragonFly BSD System Maintenance and Operation CommandsManual