Storage subsystem

In a storage subsystem having a controller equipped with a disk cache including a volatile memory and a nonvolatile memory, a battery for supplying power to the controller, and one or more storage media connected to the controller, the storage subsystem being operated using electric power supplied from an external power supply in a normal state,

• a method for controlling the storage subsystem comprising:
• using, by the controller, electric power supplied from the battery to migrate data having a high possibility of being accessed again from the host out of the data stored in the volatile memory to the nonvolatile memory when power supply from the external power supply has stopped;
• migrating, by the controller, the data having been migrated from the volatile memory to the nonvolatile memory back to the volatile memory when power supply from the external power supply has recovered;
• storing, by the controller, the data in a nonvolatile memory in the disk cache when a write data from a host is received;
• reading, by the controller, the data from the storage media and storing the same in a volatile memory in the disk cache when data corresponding to a read request from the host is not stored in the disk cache;
• returning, by the controller, read target data subjected to a read request from the host stored in the nonvolatile memory to the host if the read target data is stored in the nonvolatile memory;
• sending, by the controller, the read target data stored in the volatile memory to the host if the read target data is not stored in the nonvolatile memory and is stored in the volatile memory;
• and
• migrating, by the controller, the read target data from the nonvolatile memory to the volatile memory if the read target data stored in the nonvolatile memory is reflected in the storage media.