| The division to primary, secondary, | | | | with the write operation being much |
| tertiary and off-line storage is based | | | | slower than the read operation. Examples |
| on memory hierarchy, or distance from | | | | include CD-RW. |
| the central processing unit. There are | | | | Addressability of information |
| also other ways to characterize various | | | | In location-addressable storage, each |
| types of storage. | | | | individually accessible unit of |
| Volatility of information | | | | information in storage is selected with |
| Volatile memory requires constant power | | | | its numerical memory address. In modern |
| to maintain the stored information. | | | | computers, location-addressable storage |
| Volatile memory is typically used only | | | | usually limits to primary storage, |
| for primary storage. (Primary storage is | | | | accessed internally by computer |
| not necessarily volatile, even though | | | | programs, since location-addressability |
| today's most cost-effective primary | | | | is very efficient, but burdensome for |
| storage technologies are. Non-volatile | | | | humans. |
| technologies have been widely used for | | | | In file system storage, information is |
| primary storage in the past and may | | | | divided into files of variable length, |
| again be in the future.) | | | | and a particular file is selected with |
| Non-volatile memory will retain the | | | | human-readable directory and file names. |
| stored information even if it is not | | | | The underlying device is still |
| constantly supplied with electric power. | | | | location-addressable, but the operating |
| It is suitable for long-term storage of | | | | system of a computer provides the file |
| information, and therefore used for | | | | system abstraction to make the operation |
| secondary, tertiary, and off-line | | | | more understandable. In modern |
| storage. | | | | computers, secondary, tertiary and |
| Dynamic memory is volatile memory which | | | | off-line storage use file systems. |
| also requires that stored information is | | | | In content-addressable storage, each |
| periodically refreshed, or read and | | | | individually accessible unit of |
| rewritten without modifications. | | | | information is selected with a hash |
| Ability to access non-contiguous | | | | value, or a short identifier with |
| information | | | | number? pertaining to the memory address |
| Random access means that any location in | | | | the information is stored on. |
| storage can be accessed at any moment in | | | | Content-addressable storage can be |
| the same, usually small, amount of time. | | | | implemented using software (computer |
| This makes random access memory well | | | | program) or hardware (computer device), |
| suited for primary storage. | | | | with hardware being faster but more |
| Sequential access means that the | | | | expensive option. |
| accessing a piece of information will | | | | Capacity and performance |
| take a varying amount of time, depending | | | | Storage capacity is the total amount of |
| on which piece of information was | | | | stored information that a storage device |
| accessed last. The device may need to | | | | or medium can hold. It is expressed as a |
| seek (e.g. to position the read/write | | | | quantity of bits or bytes (e.g. 10.4 |
| head correctly), or cycle (e.g. to wait | | | | megabytes). |
| for the correct location in a revolving | | | | Storage density refers to the |
| medium to appear below the read/write | | | | compactness of stored information. It is |
| head). | | | | the storage capacity of a medium divided |
| Ability to change information | | | | with a unit of length, area or volume |
| Read/write storage, or mutable storage, | | | | (e.g. 1.2 megabytes per square |
| allows information to be overwritten at | | | | centimeter). |
| any time. A computer without some amount | | | | Latency is the time it takes to access a |
| of read/write storage for primary | | | | particular location in storage. The |
| storage purposes would be useless for | | | | relevant unit of measurement is |
| many tasks. Modern computers typically | | | | typically nanosecond for primary |
| use read/write storage also for | | | | storage, millisecond for secondary |
| secondary storage. | | | | storage, and second for tertiary |
| Read only storage retains the | | | | storage. It may make sense to separate |
| information stored at the time of | | | | read latency and write latency, and in |
| manufacture, and write once storage | | | | case of sequential access storage, |
| (WORM) allows the information to be | | | | minimum, maximum and average latency. |
| written only once at some point after | | | | Throughput is the rate at which |
| manufacture. These are called immutable | | | | information can read from or written to |
| storage. Immutable storage is used for | | | | the storage. In computer storage, |
| tertiary and off-line storage. Examples | | | | throughput is usually expressed in terms |
| include CD-R. | | | | of megabytes per second or MB/s, though |
| Slow write, fast read storage is read | | | | bit rate may also be used. As with |
| write storage which allows information | | | | latency, read rate and write rate may |
| to be overwritten multiple times, but | | | | need to be differentiated. |