Storage system for eliminating duplicated data

A storage system comprising:

• a communication interface device coupled to a communication network to which a client is coupled;
• a storage device in which a plurality of user data is stored;
• and
• a controller, which is configured to manage ranks of the plurality of user data and select comparison target data in the plurality of user data based on the ranks of the plurality of user data, wherein when the storage system receives a write request from the client, the controller is configured to:
• determine whether write target data of the write request overlaps with the selected comparison target data stored in the storage device;
• not store the write target data in the storage device and register the write target data as duplicated data when the write target data is determined to overlap with the selected comparison target data;
• and
• store the write target data in the storage device when the write target data is determined not to overlap with the selected comparison target data,
• wherein the storage system has a memory,
• wherein the selected comparison target data is stored in both the storage device and the memory,
• wherein the controller is configured to determine whether the write target data of the write request overlaps with the selected comparison target data stored in the storage device using the selected comparison target data stored in the memory,
• wherein a rank of the selected comparison target data is higher than a rank of data which is not selected in the plurality of user data, and
• wherein the controller is configured to:
• determine whether the write target data conforms with a condition of an asynchronous de-duplication process when the write target data is determined to be duplication unknown data;
• and
• store the write target data in the storage device when the write target data does not conform with the condition of the asynchronous de-duplication process.