Editing Computer data storage (section)

== Characteristics of storage ==
[[File:Samsung-1GB-DDR2-Laptop-RAM.jpg|thumb|250px|A 1&nbsp;GiB module of [[SO-DIMM|laptop]] [[DDR2 SDRAM|DDR2]] [[RAM]]]]

Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressability. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance.

{| class=wikitable style="text-align:center;"
|+ Overview
|-
! Characteristic
! [[Hard disk drive]]
! [[Optical disc]]
! [[Flash memory]]
! [[Random-access memory]]
! [[Linear Tape-Open|Linear tape-open]]
|-
! Technology
| Magnetic disk
| [[Laser]] beam
| colspan=2 | Semiconductor
| Magnetic tape
|-
! Volatility
| No
| No
| No
| Volatile
| No
|-
! Random access
| Yes
| Yes
| Yes
| Yes
| No
|-
! [[Latency_(engineering)|Latency]] (access time)
| [[Hard_disk_drive_performance_characteristics|~15 ms]] (swift)
| ~150 ms (moderate)
| None (instant)
| None (instant)
| Lack of random access (very slow)
|-
! Controller
| Internal
| External
| [[Flash memory controller|Internal]]
| Internal
| External
|-
! Failure with imminent [[data loss]]
| [[Head crash]]
| —
|colspan=2| [[Failure of electronic components|Circuitry]]
| —
|-
! Error detection
| Diagnostic ([[Self-Monitoring, Analysis and Reporting Technology|S.M.A.R.T.]])
| [[Optical_disc#Surface_error_scanning|Error rate measurement]]
| Indicated by downward spikes in transfer rates
| (Short-term storage)
| Unknown
|-
! Price per space
| Low
| Low
| High
| Very high
| Very low (but expensive drives)
|-
! Price per unit
| Moderate
| Low
| Moderate
| High
| Moderate (but expensive drives)
|-
! Main application
| Mid-term archival, routine backups, server, workstation storage expansion
| Long-term archival, [[hard copy]] distribution
| Portable electronics; operating system
| Real-time
| Long-term archival
|-
|}

=== Volatility ===
[[Non-volatile memory]] retains the stored information even if not constantly supplied with electric power. It is suitable for long-term storage of information. [[Volatile memory]] requires constant power to maintain the stored information. The fastest memory technologies are volatile ones, although that is not a universal rule. Since the primary storage is required to be very fast, it predominantly uses volatile memory.

[[Dynamic random-access memory]] is a form of volatile memory that also requires the stored information to be periodically reread and rewritten, or [[memory refresh|refreshed]], otherwise it would vanish. [[Static random-access memory]] is 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 when the power supply is lost.

An [[uninterruptible power supply]] (UPS) can be used to give a computer a brief window of time to move information from primary volatile storage into non-volatile storage before the batteries are exhausted. Some systems, for example [[EMC Symmetrix]], have integrated batteries that maintain volatile storage for several minutes.

=== 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.
; 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 [[SSD]].
; Write once storage : [[Write once read many]] (WORM) allows the information to be written only once at some point after manufacture. Examples include semiconductor [[programmable read-only memory]] and [[CD-R]].
; Read only storage : Retains the information stored at the time of manufacture. Examples include [[Read-only memory#Factory programmed|mask ROM ICs]] and [[CD-ROM]].

=== 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. Most [[semiconductor memories]], [[Flash_memory|flash memories]] and [[hard disk drive]]s provide random access, though both semiconductor and flash memories have minimal [[latency_(engineering)|latency]] when compared to hard disk drives, as no mechanical parts need to be moved.
; [[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.

=== 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 system|File addressable]]: Information is divided into ''[[computer file|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 (computer science)|abstraction]] to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems.
; [[content-addressable memory|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 [[Computer hardware|hardware]] (computer device), with hardware being faster but more expensive option. Hardware content addressable memory is often used in a computer's [[CPU cache]].

=== Capacity ===
; Raw capacity: The total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of [[bit]]s or [[byte]]s (e.g. 10.4 [[megabyte]]s).
; [[Memory storage density]]: The compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume (e.g. 1.2 megabytes per square inch).

=== Performance ===
; [[Latency (engineering)|Latency]]: The time it takes to access a particular location in storage. The relevant [[unit of measurement]] is typically [[nanosecond]] for primary storage, [[millisecond]] for secondary storage, and [[second]] for tertiary storage. It may make sense to separate read latency and write latency (especially for non-volatile memory) and in case of sequential access storage, minimum, maximum and average latency.
; [[Hard_ disk drive performance characteristics#Data transfer rate|Throughput]]: The rate at which information can be read from or written to the storage. In computer data storage, throughput is usually expressed in terms of megabytes per second (MB/s), though [[bit rate]] may also be used. As with latency, read rate and write rate may need to be differentiated. Also accessing media sequentially, as opposed to randomly, typically yields maximum throughput.
; Granularity: The size of the largest "chunk" of data that can be efficiently accessed as a single unit, e.g. without introducing additional latency.
;Reliability: The probability of spontaneous bit value change under various conditions, or overall [[failure rate]].
Utilities such as [[hdparm]] and [[sar (Unix)|sar]] can be used to measure IO performance in Linux.

=== Energy use ===
* Storage devices that reduce fan usage automatically shut-down during inactivity, and low power hard drives can reduce energy consumption by 90 percent.<ref>{{cite web|url=http://www.springlightcfl.com/consumer/energy_savings_calculator.aspx|title=Energy savings calculator|url-status=dead|archive-url=https://web.archive.org/web/20081221131054/http://springlightcfl.com/consumer/energy_savings_calculator.aspx|archive-date=21 December 2008}}</ref><ref>{{Cite web|url=http://www.simpletech.com/content/eco-friendly-redrive|url-status=dead|archive-url=https://web.archive.org/web/20080805092907/http://www.simpletech.com/content/eco-friendly-redrive|archive-date=5 August 2008|title=How much of the [re]drive is actually eco-friendly?|website=Simple tech}}</ref>
* 2.5-inch hard disk drives often consume less power than larger ones.<ref>{{cite web|title=IS the Silent PC Future 2.5-inches wide?|url=http://www.silentpcreview.com/article145-page1.html|access-date=2 August 2008|author=Mike Chin|date=8 March 2004|url-status=live|archive-url=https://web.archive.org/web/20080720000101/http://www.silentpcreview.com/article145-page1.html|archive-date=20 July 2008}}</ref><ref>{{cite web|url=http://www.silentpcreview.com/article29-page2.html|title=Recommended hard drives|access-date=2 August 2008|author=Mike Chin|date=18 September 2002|url-status=live|archive-url=https://web.archive.org/web/20080905085853/http://www.silentpcreview.com/article29-page2.html|archive-date=5 September 2008}}</ref> Low capacity [[solid-state drive]]s have no moving parts and consume less power than hard disks.<ref>{{Cite web|url=http://techreport.com/articles.x/10334/13|title=Super Talent's 2.5" IDE flash hard drive|website=The tech report|date=12 July 2006|page=13|archive-url=https://web.archive.org/web/20120126045422/http://techreport.com/articles.x/10334/13|archive-date=26 January 2012|access-date=18 June 2011}}</ref><ref>{{Cite web|url=http://www.tomshardware.com/reviews/conventional-hard-drive-obsoletism,1324-5.html|title=Power consumption – Tom's hardware : Conventional hard drive obsoletism? Samsung's 32 GB flash drive previewed|website=tomshardware.com|date=20 September 2006|access-date=18 June 2011}}</ref><ref name=xbitSSDvsHD/> Also, memory may use more power than hard disks.<ref name=xbitSSDvsHD>{{cite web|title=SSD, i-RAM and traditional hard disk drives|date=23 April 2008|url=http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|author=Aleksey Meyev|publisher=X-bit labs|url-status=dead|archive-url=https://web.archive.org/web/20081218134355/http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|archive-date=18 December 2008}}</ref> Large caches, which are used to avoid hitting the [[memory wall]], may also consume a large amount of power.

=== Security ===
[[Hardware-based full disk encryption|Full disk encryption]], [[Disk encryption software|volume and virtual disk encryption, andor file/folder encryption]] is readily available for most storage devices.<ref>{{Cite web|url=https://www.hhs.gov/sites/default/files/ocr/privacy/hipaa/administrative/securityrule/nist800111.pdf|title=Guide to storage encryption technologies for end user devices|publisher=National Institute of Standards and Technology|author1=Karen Scarfone|author2=Murugiah Souppaya |author3=Matt Sexton|date=November 2007}}</ref>

Hardware memory encryption is available in Intel Architecture, supporting Total Memory Encryption (TME) and page granular memory encryption with multiple keys (MKTME).<ref>{{cite web|url=https://software.intel.com/sites/default/files/managed/a5/16/Multi-Key-Total-Memory-Encryption-Spec.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://software.intel.com/sites/default/files/managed/a5/16/Multi-Key-Total-Memory-Encryption-Spec.pdf |archive-date=2022-10-09 |url-status=live |title=Encryption specs |publisher=software.intel.com |access-date=28 December 2019}}</ref><ref>{{cite web|url=https://lwn.net/Articles/776688/ |title=A proposed API for full-memory encryption |publisher=Lwn.net |access-date=28 December 2019}}</ref> and in [[SPARC]] M7 generation since October 2015.<ref>{{cite web|url=https://swisdev.oracle.com/_files/What-Is-SSM.html |title=Introduction to SPARC M7 and silicon secured memory (SSM)|url-status=dead|publisher=swisdev.oracle.com|archive-url=https://web.archive.org/web/20190121180236/https://swisdev.oracle.com/_files/What-Is-SSM.html|archive-date=21 January 2019|access-date=28 December 2019}}</ref>

=== Vulnerability and reliability ===
[[File:S.M.A.R.T. warning.png|thumb|S.M.A.R.T. software warning suggests impending hard drive failure.]]
Distinct types of data storage have different points of failure and various methods of [[predictive failure analysis]].

Vulnerabilities that can instantly lead to total loss are [[head crash]]ing on mechanical hard drives and [[failure of electronic components]] on flash storage.

==== Error detection ====
[[File:QPxTool DVD error rate graph.png|thumb|[[Optical_disc#Surface_error_scanning|Error rate measurement]] on a [[DVD+R]]. The minor errors are correctable and within a healthy range.]]
Impending failure on [[hard disk drive]]s is estimable using S.M.A.R.T. diagnostic data that includes the [[power-on hours|hours of operation]] and the count of spin-ups, though its reliability is disputed.<ref>{{cite web|title=What S.M.A.R.T. hard disk errors actually tell us |url=https://www.backblaze.com/blog/what-smart-stats-indicate-hard-drive-failures/|website=Backblaze|date=6 October 2016}}</ref>

Flash storage may experience downspiking transfer rates as a result of accumulating errors, which the [[flash memory controller]] attempts to correct.

The health of [[optical media]] can be determined by [[Optical disc#Surface error scanning|measuring correctable minor errors]], of which high counts signify deteriorating and/or low-quality media. Too many consecutive minor errors can lead to data corruption. Not all vendors and models of [[Optical disc drive|optical drives]] support error scanning.<ref>{{cite web |title=QPxTool - check the quality|url=https://qpxtool.sourceforge.io/faq.html|website=qpxtool.sourceforge.io}}</ref>