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