1. A method of writing data to a heterogeneous data volume (HV) having multiple disk classes of storage, the method comprising: receiving a request for a data write; setting a class of storage (CoS) tier instance to be a CoS tier of the HV that matches the data write request; determining an operating mode of the CoS tier instance; and if the operating mode comprises a static mode, then setting a CoS tier for the data write to be the CoS tier instance; and if the operating mode comprises a disabled mode, then incrementing the CoS tier instance by changing the CoS tier instance to a next lower CoS tier; and if the operating mode comprises an elastic mode, then setting the CoS tier for the data write to be the CoS tier instance with a computed probability, and otherwise incrementing the CoS tier instance by changing the CoS tier instance to the next lower CoS tier.