A storage system includes a CPU, a first memory module, a second memory module, and a storage device. The processor and the first memory module are installed in the same node. The second memory module are replaceable without shutting down power supply of the node. The first memory module stores an operating system and a program for managing user data to be stored in the storage device. The second memory module stores cache data of the user data to be stored in the storage device. The processor is configured to store a copy of data to be stored in the second memory module in the third memory module.