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