Offload defrag operation for host-managed storage

A host-managed storage device includes an offload capability that enables the host to offload all or a portion of a defrag operation to the storage device. Rather than issuing read, write or copy operations and commands to relocate data to the host's DRAM, the host assembles a defrag operation command descriptor for the storage device controller. The command descriptor includes a defrag bitmap that can be directly accessed by the storage device controller to conduct the defrag operation entirely on the storage device at band granularity, without consuming host CPU cycles or host memory. The reduction in host operations/commands achieved by offloading defragmentation to the storage device is on the order of at least a thousand-fold reduction.