The storage controller 10 determines whether the destage processing has been carried out for all the stripe rows 41 in the extent 40 or not (step S110). If the destage processing has not been carried out for all the stripe rows 41, the storage controller 10 ends the processing. If the destage processing has been carried out for all the stripe rows 41, by contrast, the storage controller 10 changes the state 523 of the relevant extent 40 in the extent state management table 520 into “CLOSE” (step S111) and ends the processing.

FIG. 13 is a flow chart for description of an example of the open processing of step S102 in FIG. 12.

The storage controller 10 checks the PG management table 530 and determines whether the data distribution state of the parity groups PG to be a storage destination of data is the first-order distribution state or not, that is, whether the data distribution state 533 indicates “FIRST ORDER” or not (step S201).