Hard disk drive high performance derives from devices with faster performance characteristics. These devices can be solid state drives (SSD) or rotating drives like optical disks, hard disk drives and floppy disk drives. A large percentage of attributes relating to movement of mechanical parts do apply to solid state drivers, however, the drive is affected by another element causing delays when measuring and isolation of the attribute. The performance characteristics can be categorized into two main categories namely: data transfer time and access time.
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The access time of a hard drive also known as response time is the total time it takes the drive to start actual transfer. The access time is affected by the mechanical make up of the drive in relation to moving heads and rotating disks. It comprises some independently measurable elements summed up to a single value during evaluation of the storage device’s performance. Manufacturers usually give an estimate value of the access time. For the solid state drivers access time depends on electrical connections to solid state memory, so their access time is very short and consistent.
Access time is obtained by adding up rotational latency, seek time, command accessing time and settle time.
This is the time it takes for the assemble head on the actuator arm to get to the track of the disk on which the data will be read or written. The data on a hard drive is stored in sectors lined up in parallel circular tracks. The actuator arm suspends a head that transfers data with the media. The average seek time is the mean of the possible seek times which technical is the time taken to do all the possible seeks dived by the total number possible seeks, but practically it’s determined by statistical methods.
Track-to-track measurement is the total time required to move from track to the next one, and is the fastest seek time. In hard disk drives the time measures 0.2-0.8 ms. Full stroke measurement is time required to move from the innermost track to the outermost track.
This is a term used to describe hard disk drives that’s purposely restricted in the total capacity so that the actuator that can only move heads accord fewer total number of total tracks. This reduces the distance the heads can be from any point on the drive; consequently reducing seek time, and the total hard drive capacity. The reduced seek time enables the hard disk drive to increase the number of IOPS that can be obtained from the drive. The power and cost per usable bite of space rises as maximum track range reduces.
Vibration control and audible noise
Audible noise (measured in dBA) is significant for a number of applications including DVRs, quiet computers and digital audio. For a disk to have minimal noise it has to use fluid bearings, reduce seek speed under load, and slower rotational speeds of approximately 5,400 rpm because these reduce clutching sound and audible clicks. Smaller drives are also quieter than larger ones.
A number of laptop- and desktop class disk drives and allow users to trade off between drive noises and seek performance. Faster seek times require more energy to quickly move the heads across the patter, and loud noises are produced by the pivot arm and greater vibrations since the heads rapidly accelerate at the onset of the seek motion and decelerate at when the motion seek ends. Quiet operation results in low movement speed and rate of acceleration, but reduces the seek performance considerably
Rotational latency can as well be called rotational latency or latency, and is when there are delays in waiting for the rotation of the disk drive to bring the disk sector requested for under the data transfer head. It depends on the speed of rotation of the disk, which is measured in revolutions per minute. Rotational latency is time taken to do a complete rotation excluding spin up time. Rotational latency and access time can be decreased through increasing the rotational speed of disks. This as well improves the throughput, which is highly beneficial for hard disk performance.
Impact of reduced power consumption
Power consumption is a critical issue for any applications. The ever increasing data center machine density has resulted in multiple problems including issues with adequate delivery of power to the devices that negatively impacting spin up, and reducing the heat generated by their components, as well as cost concerns with the environment and power. Most of today’s hard disk drives support one or more power management systems that use power modes to save energy bye optimizing performance.
When this mode is implemented a hard disk drive will switch between full power mode and other power saving modes expressed as a function of power usage. Drive manufacturers are now producing green which includes additional power saving features, but which have adverse impacts on latency.