The last layer in the memory hierarchy is magnetic tape. This medium is frequently used as a backup for disk storage and for holding very large data sets. To access a tape, it must first be put into a tape reader, either by a person or a robot (automated tape handling is common at installations with large databases). Then the tape may have to be spooled forward to get to the requested block. All in all, this could take minutes. The big plus of tape is that it is exceedingly inexpensive per bit and removable, which is important for backup tapes that must be stored off-site in order to survive fires, floods, earthquakes, and other disasters.
The memory hierarchy we have talked about is typical, but a number of installations do not have all the layers or have a few unlike ones (such as optical disk). Still, in all of them, as one goes on down the hierarchy, the random access time increases considerably, the capacity increases equally dramatically, and the cost per bit falls extremely. As a result, it is likely that memory hierarchies will be around for years to come.