6.3.3 Digital Mass Storage Systems (DMSS)  A Digital Mass Storage System refers to an IT based system that has been planned and built to be able to store and maintain large amounts of data for a given or extended period of time. These systems come in many forms; a basic DMSS could be a personal computer which has large enough hard disk drive and some kind of catalogue that can be used to keep track of the assets the system possesses. A more complex DMSS may consist of hard disk drive and/or tape storage and group of computers that control the storage entity. A DMSS can also contain many tiers of storage with different characteristics; a fast Fibre Channel based hard disk drive tier can be used to cache assets whose access time is critical while a tier built of cheaper hard disk drives could be used to hold material whose access time is not so critical, and finally tape based storage can be used as the most cost-effective tier of storage.  When a number of different storage technologies are used in a large system to build the functional entity, a HSM (Hierarchical Storage Management) system is usually deployed in such a way that it supports the different technologies working together. Larger scale systems may also be distributed geographically in order to achieve better performance and make the system more fault tolerant.