Rotational latency

Higher performance in hard disk drives comes from devices which have better performance characteristics. 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. These performance characteristics can be grouped into two categories: access time and data transfer time (or rate).

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. 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. 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:

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.

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Wikipedia