Device hierarchy is defined as levels of storage devices with each level having its different access speeds, costs-per-byte, and storage capacities.

At the top of the device hierarchy is the DASD storage control with its optional, high speed, cache storage.

In the middle of the device hierarchy is DASD. Data that needs to be accessed more frequently is typically stored on DASD.

Removable media is the lowest level in the device hierarchy. It can be used with optical and tape library data servers for the fastest response time, as well as with stand alone optical and tape drives with shelf storage. Removable media has the slowest initial access time; however, it has the lowest cost per megabyte. The storage capacity of removable media is limited only by the number of volumes your installation is willing to use and manage. In addition, volumes can be removed to a remote site to protect them from damage in the event of a disaster.