What characteristics determine the disk access speed?

Answer 1

he access time or response time of a rotating drive is a measure of the time it takes before the drive can actually transfer data. The factors that control this time on a rotating drive are mostly related to the mechanical nature of the rotating disks and moving heads. It is composed of a few independently measurable elements that are added together to get a single value when evaluating the performance of a storage device. The access time can vary significantly, so it is typically provided by manufacturers or measured in benchmarks as an average.[4][5] For SSDs this time is not dependent on moving parts, but rather electrical connections to solid state memory, so the access time is very quick and consistent.[6] Most testing and benchmark applications do not draw a distinction between rotating drives and SSDs so they both go through the same measurement process.

The key components that are typically added together to obtain the access time are:[2][7]

• Seek time
• Rotational latency
• Other
  • Command processing time
  • Settle time

Seek timeWith rotating drives, the seek time measures the time it takes the head assembly on the actuator arm to travel to the track of the disk where the data will be read or written.[7] The data on the media is stored in sectors which are arranged in parallel circular tracks (concentric or spiral depending upon the device type) and there is an actuator with an arm that suspends a head that can transfer data with that media. When the drive needs to read or write a certain sector it determines in which track the sector is located. It then uses the actuator to move the head to that particular track. If the initial location of the head was the desired track then the seek time would be zero. If the initial track was the outermost edge of the media and the desired track was at the innermost edge then the seek time would be the maximum for that drive.[8][9] Seek times are not linear compared with the seek distance traveled because of factors of acceleration and deceleration of the actuator arm.[10]

A rotating drive's average seek time is the average of all possible seek times which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice it is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks.[8][7][11] Average seek time ranges from 3 ms[12] for high-end server drives, to 15 ms for mobile drives, with the most common mobile drives at about 12 ms[13] and the most common desktop drives typically being around 9 ms.

The first HDD[14] had an average seek time of about 600 ms, and by the middle 1970s, HDDs were available with seek times of about 25 ms.[15] Some early PC drives used a stepper motor to move the heads, and as a result had seek times as slow as 80-120 ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to around 20 ms. Seek time has continued to improve slowly over time.

The other two less commonly referenced seek measurements are track-to-track and full stroke. The track-to-track measurement is the time required to move from one track to an adjacent track.[7] This is the shortest (fastest) possible seek time. In HDDs this is typically between 0.2 and 0.8 ms.[6] The full stroke measurement is the time required to move from the outermost track to the innermost track. This is the longest (slowest) possible seek time.[8]

With SSDs there are no moving parts, so a measurement of the seek time is only testing electronic circuits preparing a particular location on the memory in the storage device. Typical SSDs will have a seek time between 0.08 and 0.16 ms.[6]

Short stroking

Short stroking is a term used in enterprise storage environments to describe an HDD that is purposely restricted in total capacity so that the actuator only has to move the heads across a smaller number of total tracks. This limits the maximum distance the heads can be from any point on the drive thereby reducing its average seek time, but also restricts the total capacity of the drive. This reduced seek time enables the HDD to increase the number of IOPS available from the drive. The cost and power per usable byte of storage rises as the maximum track range is reduced, but the increase in IOPS per dollar is better.[16]

Effect of audible noise and vibration control

Measured in dBA, audible noise is significant for certain applications, such as DVRs, digital audio recording and quiet computers. Low noise disks typically use fluid bearings, slower rotational speeds (usually 5,400 rpm) and reduce the seek speed under load (AAM) to reduce audible clicks and crunching sounds. Drives in smaller form factors (e.g. 2.5 inch) are often quieter than larger drives.[17]

Some desktop- and laptop-class disk drives allow the user to make a trade-off between seek performance and drive noise. For example, Seagate offers a set of features in some drives called Sound Barrier Technology that include some user or system controlled noise and vibration reduction capability. Faster seek times typically require more energy usage to quickly move the heads across the platter, causing loud noises from the pivot bearing and greater device vibrations as the heads are rapidly accelerated during the start of the seek motion and decelerated at the end of the seek motion. Quiet operation reduces movement speed and acceleration rates, but at a cost of reduced seek performance.[18]

Rotational latencyTypical HDD figuresHDD

Spindle

[rpm]Average

rotational

latency [ms]4,2007.145,4005.567,2004.1710,0003.0015,0002.00 Comparison of several forms of disk storage showing tracks (not-to-scale); green denotes start and red denotes end.

* Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes.

Rotational latency (sometimes called rotational delay or just latency) is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head.[19] It depends on the rotational speed of a disk (or spindle motor), measured in revolutions per minute (RPM).[7][20] For most magnetic media-based drives, the average rotational latency is typically based on the empirical relation that the average latency in milliseconds for such a drive is one-half the rotational period. Maximum rotational latencyis the time it takes to do a full rotation excluding any spin-up time (as the relevant part of the disk may have just passed the head when the request arrived).[21] Therefore the rotational latency and resulting access time can be improved (decreased) by increasing the rotational speed of the disks.[7] This also has the benefit of improving (increasing) the throughput (discussed later in this article).

For more details on track layout see Disk storage

The spindle motor speed can use one of two types of disk rotation methods: 1) constant linear velocity (CLV), used mainly in optical storage, varies the rotational speed of the optical disc depending upon the position of the head, and 2) constant angular velocity (CAV), used in HDDs, standard FDDs, a few optical disc systems, and vinyl audio records, spins the media at one constant speed regardless of where the head is positioned.

Another wrinkle occurs depending on whether surface bit densities are constant. Usually, with a CAV spin rate, the densities are not constant so that the long outside tracks have the same number of bits as the shorter inside tracks. When the bit density is constant, outside tracks have more bits than inside tracks and is generally combined with a CLV spin rate. In both these schemes contiguous bit transfer rates are constant. This is not the case with other schemes such as using constant bit density with a CAV spin rate.

Effect of reduced power consumption

Power consumption has become increasingly important, not only in mobile devices such as laptops but also in server and desktop markets. Increasing data center machine density has led to problems delivering sufficient power to devices (especially for spin-up), and getting rid of the waste heat subsequently produced, as well as environmental and electrical cost concerns (see green computing). Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance. When implemented, an HDD will change between a full power mode to one or more power saving modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep where the drive is stopped or spun down, may take as long as several seconds to be fully operational thereby increasing the resulting latency.[22] The drive manufacturers are also now producing green drives that include some additional features that do reduce power, but can can adversely affect the latency including slower spindle speeds and parking heads off the media to reduce friction.[23]