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 ( to ).
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
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!
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
Another classification: Primary, secondary, and tertiary storage.
Primary storage: the fastest storage media, such as cash and main memory.
Secondary (or on-line) storage: the next level of the hierarchy,
e.g., magnetic disks.
Tertiary (or off-line) storage: magnetic tapes and optical disk juke boxes.
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.