physical storage devices

1. Several types of data storage exist in most computer systems. They vary in speed of access, cost per unit of data, and reliability.

* Cache: most costly and fastest form of storage. Usually very small, and managed by the operating system.

* Main Memory (MM): the storage area for data available to be operated on.

• General-purpose machine instructions operate on main memory.
• Contents of main memory are usually lost in a power failure or “crash”.
• Usually too small (even with megabytes) and too expensive to store the entire database.

* Flash memory: EEPROM (electrically erasable programmable read-only memory).

• Data in flash memory survive from power failure.
• Reading data from flash memory takes about 10 nano-secs (roughly as fast as from main memory), and writing data into flash memory is more complicated: write-once takes about 4-10 microsecs.
• To overwrite what has been written, one has to first erase the entire bank of the memory. It may support only a limited number of erase cycles ( tex2html_wrap_inline570 to tex2html_wrap_inline572 ).
• It has found its popularity as a replacement for disks for storing small volumes of data (5-10 megabytes).

* Magnetic-disk storage: primary medium for long-term storage.

• Typically the entire database is stored on disk.
• Data must be moved from disk to main memory in order for the data to be operated on.
• After operations are performed, data must be copied back to disk if any changes were made.
• Disk storage is called direct access storage as it is possible to read data on the disk in any order (unlike sequential access).
• Disk storage usually survives power failures and system crashes.

* Optical storage: CD-ROM (compact-disk read-only memory), WORM (write-once read-many) disk (for archival storage of data), and Juke box (containing a few drives and numerous disks loaded on demand).

* Tape Storage: used primarily for backup and archival data.

• Cheaper, but much slower access, since tape must be read sequentially from the beginning.
• Used as protection from disk failures!

2. The storage device hierarchy is presented in Figure 10.1, where the higher levels are expensive (cost per bit), fast (access time), but the capacity is smaller.

Figure 10.1:   Storage-device hierarchy

3. Another classification: Primary, secondary, and tertiary storage.
1. Primary storage: the fastest storage media, such as cash and main memory.
2. Secondary (or on-line) storage: the next level of the hierarchy, e.g., magnetic disks.
3. Tertiary (or off-line) storage: magnetic tapes and optical disk juke boxes.
4. Volatility of storage. Volatile storage loses its contents when the power is removed. Without power backup, data in the volatile storage (the part of the hierarchy from main memory up) must be written to nonvolatile storage for safekeeping.