Disk-drive performance characteristics

Disk-drive performance characteristics are the attributes which control the time it takes to transfer (read or write) data between a computer and a data storage device (most typically disk storage) starting with the initial command from the computer or host until the storage device completes the command. Higher performance comes from devices which have faster performance characteristics.^[1][2] These devices include those with rotating media, hereby called rotating drives, i.e., hard-disk drives (HDD), floppy disk drives (FDD), optical discs (DVD-RW / CD-RW), and it also covers devices without moving parts like solid-state drives (SSD). For SSDs, most of the attributes related to the movement of mechanical components are not applicable, but the device is actually affected by some other electrically based element that still causes a measurable delay when isolating and measuring that attribute.^[3] These performance characteristics can be grouped into two categories: access time and data transfer time (or rate).^[4]

Access time

A hard disk head on an access arm resting on a hard disk platter.

The 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 time

With 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 latency

Typical HDD figures

HDD Spindle [rpm]	Average rotational latency [ms]
4,200	7.14
5,400	5.56
7,200	4.17
10,000	3.00
15,000	2.00

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 latency is 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]

Other

The command processing time or command overhead is the time it takes for the drive electronics to set up the necessary communication between the various components in the device so it can read or write the data. This is in the range of 0.003 ms. With a value this low most people or benchmarks tend to ignore this time.^[2][24]

The settle time measures the time it takes the heads to settle on the target track and stop vibrating so it does not read or write off track. This amount is usually very small (typically less than 0.1 ms) or already included in the seek time specifications from the drive manufacturer.^[25] In a benchmark test the settle time would be included in the seek time.

Data transfer rate

The data transfer rate of a drive (also called throughput) covers both the internal rate (moving data between the disk surface and the controller on the drive) and the external rate (moving data between the controller on the drive and the host system). The measurable data transfer rate will be the lower (slower) of the two rates. The sustained data transfer rate or sustained throughput of a drive will be the slower of the sustained internal and sustained external rates. The sustained rate is less than or equal to the maximum or burst rate because it does not have the benefit of any cache or buffer memory in the drive. The internal rate is further broken down into media rate, head switch time, and cylinder switch time. These are not applicable to SSDs.^[7][26]

• Media rate – speed at which the drive can read bits from the surface of the media
• Head switch time – time required to electrically switch from one head to another; only applies to multi-head drive and is about 1 to 2 ms.^[27]
• Cylinder switch time – time required to move to an adjacent track; the name cylinder is used because typically all the tracks of a drive with more than one head or data surface are read before moving the actuator. This time is typically about twice the track-to-track seek time or about 2 to 3 ms.^[28]

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. As of 2010^[update], a typical 7200 RPM desktop HDD has a sustained "disk-to-buffer" data transfer rate up to 1030 Mbit/s.^[29] This rate depends on the track location, so it will be higher for data on the outer tracks (where there are more data sectors) and lower toward the inner tracks (where there are fewer data sectors); and is generally somewhat higher for 10,000 RPM drives. A current widely used standard for the "buffer-to-computer" interface is 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 disk-to-buffer transfer rates.

SSDs do not have the same internal limits of HDDs, so their internal and external transfer rates are often maximizing the capabilities of the drive-to-host interface.

Effect of file fragmentation

Transfer rate can be influenced by file system fragmentation and the layout of the files. Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.^[30] Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, the procedure can slow response when performed while the computer is in use.^[31]

In stark contrast to HDDs, flash memory-based SSDs do not need defragmentation. The nature of recording information on flash memory wears it out over time, so any unnecessary writes to the SSD is bad. Since the data is accessed differently (solid state electronics compared to physical sectors on a disk) defragmentation is not necessary or desirable.^[32]

Effect of areal density

HDD data transfer rate depends upon the rotational speed of the disks and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates.^[33] Areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, the latter will increase the data transfer rate (for a given RPM). Improvement of data transfer rate performance is correlated to the areal density only by increasing a track's linear surface bit density (sectors per track). Simply increasing the number of tracks on a disk can affect seek times but not gross transfer rates. Based on historic trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year.^[34] Seek times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

References

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