Editing Hard disk drive (section)

== Performance characteristics ==
{{Main|Hard disk drive performance characteristics}}
The factors that limit the [[access time|time to access the data]] on an HDD are mostly related to the mechanical nature of the rotating disks and moving heads, including:
* [[Hard disk drive performance characteristics#Seek time|Seek time]] is a measure of how long it takes the head assembly to travel to the track of the disk that contains data. 
* Rotational latency is incurred because the desired [[disk sector]] may not be directly under the head when data transfer is requested. Average rotational latency is shown in the table, based on the statistical relation that the average latency is one-half the rotational period.
* The [[bit rate]] or data transfer rate (once the head is in the right position) creates delay which is a function of the number of blocks transferred; typically relatively small, but can be quite long with the transfer of large contiguous files.

Delay may also occur if the drive disks are stopped to save energy.

[[Defragmentation]] is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.<ref name="itworld-2001-04-18" /> Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the procedure is in progress.<ref name="AutoMK-67" />

Time to access data can be improved by increasing rotational speed (thus reducing latency) or by reducing the time spent seeking. Increasing areal density increases [[throughput]] by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. The time to access data has not kept up with throughput increases, which themselves have not kept up with growth in bit density and storage capacity.

=== Latency ===
{| class="wikitable" style="margin-left: 1.5em;"
|+Latency characteristics typical of HDDs
|-
! Rotational speed (rpm)
! Average rotational latency (ms){{Efn|Average rotational latency in milliseconds is computed as follows: 60&nbsp;× 1000&nbsp;÷ 2&nbsp;÷ R, where {{mvar|R}} is rotational speed revolutions per minute.}}
|-
| 15,000
| 2
|-
| 10,000
| 3
|-
| 7,200
| 4.16
|-
| 5,400
| 5.55
|-
| 4,800
| 6.25
|}

=== Data transfer rate ===
{{As of|2010}}, a typical 7,200-rpm desktop HDD has a sustained "disk-to-[[disk buffer|buffer]]" data transfer rate up to {{nowrap|1,030 Mbit/s}}.<ref name="AutoMK-71" /> This rate depends on the track location; the rate is higher for data on the outer tracks (where there are more data sectors per rotation) and lower toward the inner tracks (where there are fewer data sectors per rotation); and is generally somewhat higher for 10,000-rpm drives. A current, widely used standard for the "buffer-to-computer" interface is {{nowrap|3.0 Gbit/s}} SATA, which can send about 300 megabyte/s (10-bit encoding) from the buffer to the computer, and thus is still comfortably ahead of today's{{As of?|date=September 2023}} disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file-generator tools, then reading back the file. Transfer rate can be influenced by [[file system fragmentation]] and the layout of the files.<ref name="itworld-2001-04-18" />

HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates. Higher speeds require a more powerful spindle motor, which creates more heat. While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track,<ref>{{cite web |title=GLOSSARY of DRIVE and COMPUTER TERMS |url=http://ftp.seagate.com/techsuppt/misc/glossary.txt |publisher=Seagate |access-date=August 4, 2018}}</ref> only the latter increases the data transfer rate for a given rpm. Since data transfer rate performance tracks only one of the two components of areal density, its performance improves at a lower rate.<ref>{{cite journal |title=Bit Patterned Magnetic Recording: Theory, Media Fabrication, and Recording Performance |journal = IEEE Transactions on Magnetics|volume = 51|issue = 5|pages = 1–42|publisher=HGST, a Western Digital Company |arxiv = 1503.06664|doi = 10.1109/TMAG.2015.2397880|year = 2015|last1 = Albrecht|first1 = Thomas R.|last2 = Arora|first2 = Hitesh|last3 = Ayanoor-Vitikkate|first3 = Vipin|last4 = Beaujour|first4 = Jean-Marc|last5 = Bedau|first5 = Daniel|last6 = Berman|first6 = David|last7 = Bogdanov|first7 = Alexei L.|last8 = Chapuis|first8 = Yves-Andre|last9 = Cushen|first9 = Julia|last10 = Dobisz|first10 = Elizabeth E.|last11 = Doerk|first11 = Gregory|last12 = He Gao|last13 = Grobis|first13 = Michael|last14 = Gurney|first14 = Bruce|last15 = Hanson|first15 = Weldon|last16 = Hellwig|first16 = Olav|last17 = Hirano|first17 = Toshiki|last18 = Jubert|first18 = Pierre-Olivier|last19 = Kercher|first19 = Dan|last20 = Lille|first20 = Jeffrey|last21 = Zuwei Liu|last22 = Mate|first22 = C. Mathew|last23 = Obukhov|first23 = Yuri|last24 = Patel|first24 = Kanaiyalal C.|last25 = Rubin|first25 = Kurt|last26 = Ruiz|first26 = Ricardo|last27 = Schabes|first27 = Manfred|last28 = Lei Wan|last29 = Weller|first29 = Dieter|last30 = Tsai-Wei Wu|display-authors = 29|bibcode = 2015ITM....5197880A|s2cid = 33974771}}</ref>

=== Other considerations ===
Other performance considerations include quality-adjusted [[price index|price]], power consumption, audible noise, and both operating and non-operating shock resistance.