A computer in a disk node executes a data management program. A deduplication-eligible data unit detection module detects a data unit whose deduplication grace period after last write time has expired. A deduplication address fetch module interacts with an index server to obtain a deduplication address associated with a unique value of data stored in a constituent storage space allocated to the data unit that is found to be deduplication-eligible. A constituent storage space deallocation module stores the obtained deduplication address in a data unit record memory, together with information indicating the detected data unit. Simultaneously a constituent storage space deallocation module releases the allocated constituent storage space from the detected data unit.