Cache memories is small; its use is managed by the computer system appliance. We shall not be concerned about controlling cache storage in the database system. The contents of main memory are usually missing if a power failure or system crash occurs. A drawback of flash memory is that it can support only a limited number of erase cycles, ranging from 10000 to 1 million. The size of magnetic tapes currently ranges from a few gigabytes to 80 gigabytes. Both the lower and upper end of this range are actually growing at about 50 percent per year, and we can expect much larger capacity disks every year.