Computer system and storage control method

A computer system, comprising:

• one or more host computers configured to transmit an Input/Output (I/O) request;
• and
• one or more storage apparatuses configured to:
• receive the I/O request transmitted from the host computer;
• and
• process the I/O request,
• each storage apparatus comprising:
• a first storage device;
• a memory, including a main cache that is a cache memory region configured to temporarily store data that is inputted/outputted to/from the first storage device in response to the received I/O request;
• and
• a first control device coupled to the first storage device and the memory,
• the host computer comprising:
• a second storage device;
• and
• a second control device coupled to the second storage device,
• wherein an entirety or a part of free space of the second storage device is an external cache that is a region used as a cache memory region outside of the storage apparatus,
• wherein:
• (A) if the I/O is a write, the I/O request is a write request, and the first control device is configured to:
• Write data associated with the write request into the main cache, and
• write the data in the main cache into the first storage device,
• (B) the first control device is configured to write the data in the main cache into the external cache, and
• (C) if the I/O is a read, the I/O request is a read request;
• (c1) the second control device is configured to determine whether read data as target data of the read request exists in the external cache, and
• (c2) if a result of the determination in (c1) is positive, the second control device is configured to read the read data from the external cache;
• and
• wherein the first control device is configured to delete data from the first storage device such that a total data size of the first storage device is smaller than a cache use size that is determined on a basis of a working state of the host computer and is used as the external cache.