In a state t3 after the addition of the SSD 20, the storage controller 10 carries out first data distribution processing in which data stored in the SSD block 21 corresponding to any of the entries 42 in each stripe row 41 in the extent 40 is moved to an SSD block 21 in the added SSD 20b. Movement of the data is carried out through (a) copying of the data stored in the source SSD block 21 into the SSD block 21 in the added SSD 20b, (b) update of correspondence between the extent 40 and the SSD blocks 21, and (c) liberation of the source SSD block 21, for instance.

It is then desirable for the storage controller 10 to move the user data and not to move the redundant data. The storage controller 10 may move the data while periodically changing the source SSD 20a for each stripe row 41 so that the number of liberated SSD blocks 21 in each of the SSDs 20 may be equalized, for instance.

In the state t3, the RAID configuration in the physical parity group PG becomes 3D+1P, while the RAID configuration on the logical extent 40 remains 2D+1P adopted before the addition of the SSD 20. Therefore, the data may be distributed among both of the SSDs 20a that are the existing SSDs 20 and the SSD 20b that is the added SSD 20, without restructuring of the data such as repartition of the data or regeneration of the redundant data.