A system comprising: a plurality of storage devices, each of which includes one or more non-volatile memory chips and a device controller coupled to the non-volatile memory chips and a system controller configured to control the plurality of storage devices as a RAID group, the plurality of storage devices including a first storage device (50, data FMPK) storing old data and a second storage device (50, parity FMPK) storing old parity corresponding to the old data, wherein the system controller is configured to transfer, when receiving new data for updating the old data in accordance with a write command, the new data to a first device controller (60, data FMPK) in the first storage device (50, data FMPK) characterized in that the system controller is further configured to keep, when receiving the new data for updating the old data in accordance with the write command, the old data in a valid state, the first device controller (60, data FMPK)is configured to generate an intermediate parity based on the old data and the received new data which updates the old data and to transfer the generated intermediate parity to the system controller, the system controller is configured to send the received intermediate parity in accordance with issuing of a parity-update-write command to the second storage device (50, parity FMPK), a second device controller (60, parity FMPK) in the second storage device (50, parity FMPK) is configured to receive the intermediate parity for generating a new parity based on the old parity and the intermediate parity, the system controller is configured to issue, after receiving a completion response regarding the parity-update write command based on the new parity from the second storage device (50, parity FMPK), an unmap-old-data command for invalidating the old data, and the first device controller (60, data FMPK) in the first storage device (50, data FMPK) is configured to invalidate, after receiving the unmap-old-data command, the old data which has been kept in the first storage device (50, data FMPK) in a valid state. a plurality of storage devices, each of which includes one or more non-volatile memory chips and a device controller coupled to the non-volatile memory chips and a system controller configured to control the plurality of storage devices as a RAID group, the plurality of storage devices including a first storage device (50, data FMPK) storing old data and a second storage device (50, parity FMPK) storing old parity corresponding to the old data, wherein the system controller is configured to transfer, when receiving new data for updating the old data in accordance with a write command, the new data to a first device controller (60, data FMPK) in the first storage device (50, data FMPK) characterized in that the system controller is further configured to keep, when receiving the new data for updating the old data in accordance with the write command, the old data in a valid state, the first device controller (60, data FMPK)is configured to generate an intermediate parity based on the old data and the received new data which updates the old data and to transfer the generated intermediate parity to the system controller, the system controller is configured to send the received intermediate parity in accordance with issuing of a parity-update-write command to the second storage device (50, parity FMPK), a second device controller (60, parity FMPK) in the second storage device (50, parity FMPK) is configured to receive the intermediate parity for generating a new parity based on the old parity and the intermediate parity, the system controller is configured to issue, after receiving a completion response regarding the parity-update write command based on the new parity from the second storage device (50, parity FMPK), an unmap-old-data command for invalidating the old data, and the first device controller (60, data FMPK) in the first storage device (50, data FMPK) is configured to invalidate, after receiving the unmap-old-data command, the old data which has been kept in the first storage device (50, data FMPK) in a valid state.