Higher performance in hard disk drives comes from devices which have better performance characteristics. These performance characteristics can be grouped into two categories: access time and data transfer time (or rate).
Access time
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.
The key components that are typically added together to obtain the access time are:
Seek time
Rotational latency
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. 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. Seek times are not linear compared with the seek distance traveled because of factors of acceleration and deceleration of the actuator arm.
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.
Seek times & characteristics
The first HDD had an average seek time of about 600 ms. and by the middle 1970s, HDDs were available with seek times of about 25 ms. 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 fastest high-end server drives today have a seek time around 4 ms. Some mobile devices have 15 ms drives, with the most common mobile drives at about 12 ms and the most common desktop drives typically being around 9 ms.
Two other 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. This is the shortest (fastest) possible seek time. In HDDs this is typically between 0.2 and 0.8 ms. 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.
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.
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, lower 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.