Recently, a SSD (Solid State Drive) mounted with a large-capacity flash memory has been put to practical use.
A technique has been developed for a storage apparatus mounted with a HDD (Hard Disk Drive) mixed with a SSD different in write performance of write data from a cache memory. With this technique, an SSD achieving inferior write performance is provided with a cache memory separately from a cache memory for the HDD (refer to PTL 1, for example).