Storage system and data difference management method in storage system

A data difference management method for a storage system including four or more nodes each having a processor, a memory, and a storage drive, wherein

• for user data stored in the storage drive, a redundancy group including a plurality of data blocks of the user data and a first parity and a second parity based on the data blocks is configured,
• for each of the plurality of nodes, the storage drive of the corresponding node includes a user area that stores the plurality of data blocks belonging to different ones of the redundancy groups and a parity area that stores the second parity, and
• the method causes the processor to perform processing of:
• generating the second parity stored in the parity area of the corresponding node based on the first parity generated based on the plurality of data blocks belonging to the different redundancy groups stored in the user area of one of the nodes other than the corresponding node and on the plurality of data blocks belonging to an identical one of the redundancy groups stored in a distributed manner in the user areas of the plurality of nodes excluding the corresponding node and the one node;
• managing difference information, by associating the plurality of data blocks stored in the user area and the first parity and the second parity stored in the parity area of the corresponding node, the difference information indicating presence or absence of a difference related to update of one or both of the data block and the second parity both belonging to the identical redundancy group with each of the data blocks, the first parity, and the second parity of the corresponding node;
• and
• managing, in a case where the data block stored in the user area of the corresponding node is updated during a period in which the corresponding node is closed, the difference information related to the update in one node that is not closed and is normally operating out of two nodes other than the corresponding node.