Ch11 Storage and File Structure

11.1 Overview of Physical Storage Media

Storage media are classified by the speed with which data can be accessed, by the cost per unit of data to buy the medium, and by the medium's reliability.

• Cache:fastest,most costly. small. Its use is managed by the computer system hardware.
• Main memory: The general-purpose machine instructions operate on main memory. It is generally too small for storing the entire database. The contents of main memory are usually lost if a power failure or system crash occurs.
• Flash memory. Flash memory differs from main memory is that data survive power failure. Reading data from flash memory takes less than 100 nanoseconds, which is roughly as fast as reading data from main memory. Writing data to flash memory is more complicated--data can be written once, which takes about 4 to 10 microseconds, but cannot be overwritten directly. It should first be erased. A drawback of flash memory is that it can support only a limited number of erase cycles, ranging from 10,000 to 1 million.
• Magnetic-disk storage. The primary medium for the long-term on-line storage of data is the magnetic disk.