1. A cache disk storage, comprising: a plurality of disk drive subunits configured to store data, and each disk drive subunit having a maximum capacity, the sum of said disk drive subunits comprising a cache maximum capacity; and a cache control configured to establish a capacity restriction applicable to a filesystem providing active perceived available capacity spanning said plurality of disk drive subunits, said perceived available capacity of each said disk drive subunit in substantial proportion to said disk drive subunit maximum capacity as with respect to the relationship between the total amount of active perceived available capacity of data storage provided by said filesystem in said cache disk storage and said cache maximum capacity, and to respond to a requested upgrade adjustment of available capacity of said cache disk storage, said cache control establishing a new capacity restriction applicable to said filesystem without adding disk drive subunits, said new capacity restriction providing active perceived available capacity spanning said plurality of disk drive subunits, said perceived available capacity of each said disk drive subunit in substantial proportion to said disk drive subunit maximum capacity as with respect to the relationship between the total amount of active perceived available capacity of data storage provided by said filesystem in said cache disk storage and said cache maximum capacity, said new capacity restriction related to said requested upgrade adjustment, said new capacity restriction controlling rates of data transfer with respect to said filesystem.