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

A hammer is a tool that delivers a blow (a sudden impact) to an object. Most hammers are hand tools used to drive nails, fit parts, forge metal, and break apart objects. Hammers vary in shape, size, and structure, depending on their purposes. To break apart objects, the Bussle of the hammer is used.