Computer storage

Categories: Cleanup from August 2005 | Computer storage | Digital electronics | Information technology

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The term storage refers to the parts of a computer that retain physical state (data) for some interval of time, possibly even after electrical power to the computer is turned off. "Storage" is generally used for the slower forms.

In a home computer, storage will often take the form of:

A hard disk, which stores the user's files and programs. It keeps data even if the computer is turned off, and has a large capacity.
different sorts of drives
tertiary storage, using robotic arms

1 Different types and different purposes
2 A list of storage devices
3 See also

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Different types and different purposes

There are many ways in which types of memory (fast access) or storage (slower access) can be categorised. These include the following technologies:

primary access from the CPU or secondary (indirect) access by the CPU, which was based primarily on speed of access to the memory.
Random-Access or Sequential-Access, which is based on the mechanism of reading the memory.
Mutable (Read-write) vs. Immutable (Read only) vs. Write-Once-Read-Many (WORM) storage.
Block or File access.
media categories, for example, semiconductor storage, optical storage, phase change, magneto-optical storage, and magnetic storage, which is mainly disk-based, rather than memory-based.

Each type of storage is suited for different purposes, and most computers contain several types: primary, secondary, and volatile.

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Primary vs. secondary storage

In traditional parlance, primary storage contains data that are actively being used (for example, the programs currently being run and the data they are operating on). It is typically high-speed, relatively small, is often (but not always) volatile. It is sometimes referred to as "Main Memory." It can be accessed immediately and randomly.

Secondary storage, also known as peripheral storage, is where the computer stores information that is not necessarily in current use. It is typically slower and higher-capacity than primary storage. It is almost always non-volatile. It is slow due to serial access(thus it is also termed Serial Access Memory).

Confusingly, these terms are sometimes used differently. Primary storage can be used to refer to local random-access disk storage, which should properly be called secondary storage. If this type of storage is called primary storage, then the term secondary storage would refer to offline, sequential-access storage like tape media. This usage usually occurs in contexts where only the slower, larger forms of storage are being discussed.

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Mutable vs. immutable storage

Data stored in mutable storage can be overwritten at any time. Data stored in immutable storage cannot be overwritten. Systems can be made more secure by storing programs and static data in immutable storage, where they cannot be changed by an attacker. Dynamic data is stored in mutable storage because it must be changed from time to time. Most operating systems store all programs and data on hard disk drives, which are inherently mutable storage devices. File system permissions can be used to make certain areas of the hard disk logically immutable. However, the superuser is normally not affected by these permissions thus allowing some attacks to succeed.

Some operating systems, such as Linux, extend this logical immutability so data remains immutable even if an attacker gains superuser access. Attackers may be able to destroy the data but they can't change it.

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Block vs. file access

In disk storage, these are the two primary access methods. Block access means that the disk is divided into normally equal-sized blocks which are accessed at random by the operating system. File access contains an abstraction of files and directories which can be used to refer to storage content. Another access method, content-addressable storage (CAS) uses a hashing algorithm to refer to pieces of data.

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A list of storage devices

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Computer storage

Contents

Different types and different purposes

Primary vs. secondary storage

Mutable vs. immutable storage

Block vs. file access

A list of storage devices

See also