Removable storage devices, like tapes or disks, can store multiple programs and related data in the same device. And generally speaking, libraries are collections of physical removable media, such as tapes. Media pools organize the data on that media logically according to applications. Since a single tape is likely to contain data from many programs, media pools can be invaluable in categorizing and separating data on removable storage. Media pools provide a way to organize that removable storage by type (tape, disk, etc.) and by application.
The two other types of media pools are unrecognized media pools, which consist of media the system doesn't recognize, usually because it hasn't been registered with the system; and import media pools, which contains media the system recognizes, but which hasn't been used before by a particular removable storage system. Typically, imported media will be tapes or disks from another removable storage system, such as another office. Before either type can be used, they must be moved to a free or application media pool.
To get the most out of media pools, you need to consider how you're going to organize your removable storage. The hardest part of organizing your media pools is deciding which applications to put in specific pools and how much storage to allocate to each. If you have several kinds of media, such as removable disks as well as tapes, you also need to decide which kinds of storage to use for which applications. Usually, you will want to match the storage media characteristics, such as cost, retrieval speed and so forth, to the needs of a particular application in your organization. The only complicating factor is that you cannot have more than one kind of media in each media pool. Thus, you can't do something like split an application's storage between removable disk and tape without setting up more than one media pool for that application.
Removable storage gives you the option of automatically drawing media from the free media pool as needed. Most of the time this is the preferred option, although you can require that media be specifically allocated if you're especially concerned about the growth of removable media use. If you do decide to automatically allocate media, you can limit how much storage will be allocated to a given media pool.
Generally, it is best to set removable media to return media to the free media pool when it is no longer needed.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.