Systems, methods, and computer program products for distributed data storage. A spot defragmentation method commences upon receiving an incoming storage I/O command to access a subject logical object that is composed of subject logical data blocks. The method continues by retrieving a block map that characterizes spatial relationships between the subject logical data blocks and instances of respective subject physical data blocks that store the subject logical object on a storage device. During processing of the incoming storage I/O command, the method determines occurrences of, and locations of one or more fragmented physical data blocks. A defragmentation operation is initiated to coalesce the fragmented physical data blocks to another location. The defragmentation operation is initiated before completing the storage I/O command. In some instances, defragmentation operations are performed over a snapshot file such that upon completing the defragmentation operations and/or spot coalesce operations, the snapshot file can be deleted.