Characteristics of storage
Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics. These core characteristics are volatility, mutability, accessibility, and addressability.
1. Volatility
Non-volatile memory
Will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information. Nowadays used for most of secondary, tertiary, and off-line storage..
Volatile memory
Requires constant power to maintain the stored information. The fastest memory technologies of today are volatile ones. Since primary storage is required to be very fast, it predominantly uses volatile memory.
2. Differentiation
Dynamic random access memory
A form of volatile memory, which also requires the stored information to be periodically re-read and re-written, or refreshed, otherwise it would vanish.
Static memory
A form of volatile memory similar to DRAM with the exception that it never needs to be refreshed as long as power is applied. (It loses its content if power is removed).
3. Mutability
Read/write storage or mutable storage
Allows information to be overwritten at any time. A computer without some amount of read/write storage for primary storage purposes would be useless for many tasks. Modern computers typically use read/write storage also for secondary storage.
Read only storage
Retains the information stored at the time of manufacture, and write once storage (Write Once Read Many) allows the information to be written only once at some point after manufacture. These are called immutable storage. Immutable storage is used for tertiary and off-line storage. Examples include CD-ROM and CD-R
Slow write, fast read storage
Read/write storage, which allows information to be overwritten multiple times, but with the write operation being much slower than the read operation. Examples include CD-RW and flash memory.
4. Accessibility
Random access
Any location in storage can be accessed at any moment in approximately the same amount of time. Such characteristic is well suited for primary and secondary storage.
Sequential access
The accessing of pieces of information will be in a serial order, one after the other; therefore the time to access a particular piece of information depends upon which piece of information was last accessed. Such characteristic is typical of off-line storage.
5. Addressability
Location-addressable
Each individually accessible unit of information in storage is selected with its numerical memory address. In modern computers, location-addressable storage usually limits to primary storage, accessed internally by computer programs, since location-addressability is very efficient, but burdensome for humans.
File addressable
Information is divided into files of variable length, and a particular file is selected with human-readable directory and file names. The underlying device is still location-addressable, but the operating system of a computer provides the file system abstraction to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems.
Content-addressable
Each individually accessible unit of information is selected based on the basis of (part of) the contents stored there. Content-addressable storage can be implemented using software (computer program) or hardware (computer device), with hardware being faster but more expensive option. Hardware content addressable memory is often used in a computer's CPU cache.
