Upgrades are made to the available capacity of cache disk storage without adding drawer(s) or blade(s) when the upgrade is requested. A cache control is configured to respond to a requested capacity upgrade, establishing a capacity restriction applicable to a filesystem providing active perceived available capacity spanning the disk drive subunits, such as drawers, blades or disk drives. The active perceived available data storage capacity and unused capacity of the filesystem are spread across the disk drive subunits. The capacity restrict ion is related to the requested upgrade adjustment, and the capacity restriction controls rates of data transfer with respect to the filesystem. The capacity restriction may comprise a freespace threshold for throttling of a fill rate of resident data pending premigration from the cache disk storage filesystem, and a premigration threshold for increasing drain rate of premigration data from the cache disk storage filesystem.