HAMMERbestandssysteem

HAMMERbestandssysteem, commonly referred to as HAMMER, is a file system designed for DragonFly BSD. Introduced by Matthew Dillon, it targets data integrity, scalability, and advanced backup capabilities. The design relies on copy-on-write for both metadata and data blocks, enabling robust crash recovery and efficient snapshots. Updates are performed within atomic transactions, ensuring filesystem consistency even after power failures.

Its architecture centers on a metadata hierarchy organized for fast lookup, with checksums protecting both metadata and data blocks. HAMMER supports spanning a filesystem across multiple devices and managing very large volumes by dynamically allocating space. It includes live snapshot support, allowing administrators to capture a consistent view of the filesystem at a point in time without stopping I/O. Snapshots are space-efficient because unchanged data blocks are shared between the live filesystem and older snapshots. The system also provides means to roll back to a snapshot or access historical data for restores and backups.

Administrators can perform integrity verification and recovery using built-in tooling; HAMMER is designed to withstand disk failures and provide rapid recovery paths. The project has evolved, and a later DragonFly BSD development introduced HAMMER2 as an improved successor with different design choices; HAMMER remains documented for historical reference and in some deployments where backward compatibility is important.