Compound storage system

A compound storage system comprising:

• a storage box having a plurality of storage devices;
• and
• a plurality of servers capable of executing one or more virtual machines,
• wherein the storage box stores a logical volume,
• the virtual machines executable by the server include an application virtual machine (VM) that executes an application using the logical volume, and a controller VM that configures a controller capable of controlling read and write with respect to a logical volume for which the controller has a control right,
• when a predetermined situation occurs in which an application VM of a migration source server is migrated to a predetermined migration destination server,
• at least one processor of the one or more servers in the compound storage system is configured to:
• migrate the application VM to the migration destination server,
• migrate a control right of a logical volume used by the application VM to a controller VM of the migration destination server, and
• transfer cache data stored in a cache memory of a controller VM of the migration source server of a logical volume used by the application VM to a cache memory of a controller VM of the migration destination server.