HDD Performance

The performance of the disk system depends on the kinematics speed of the HDD. Mechanical moving details for the present remain the slowest link in a chain of data transfer from the magnetic surface of a disk to the computer random access memory. The most durational phases in data reading/data recording operations are:

  • Search for the track and reading out the several servo marks for the exact positioning of a magnetic head on the track containing the required sector.
  • Waiting for disk turning by the angle necessary for accessing the sector of the identified track (half of the turn of a magnetic disk at average).
Velocity characteristics of a hard disk drive are usually defined by two parameters:
  • Average access time (the result of dividing the time required for a series of readings of the casual sector by the number of reading sectors).
  • Average read speed (the number of sectors, read out consecutively from magnetic disk surface for a specified period).

However, the additional parameters providing a more precise determination of the performance of the drive system, on the whole, are also often used:

  • Buffered read speed (speed of information exchange between the controller of the motherboard and the hard drive controller).
  • Steady read speed (the most frequently repeating speed at the consecutive reading of the same information blocks).

The increase of speed of moving of magnetic heads is limited by the inertness of a rather massive system of positioning and the negative vibration occurring at fast chaotic (unbalanced) back-and-forth motions of mechanical components of a hard drive. Therefore, an increase in the speed of rotation of a magnetic disk became the main way of improving the performance of the storage devices evolution. It reduces the time of waiting for the sector and increases the speed of linear reading. The speed off to realize the potential of the disk drive completely reading also increases at the increase in record density and removal of the track from the center of rotation of a magnetic disk. The usage of the automatic acoustic management (AAM) technology in hard drives allows controlling the speed of positioning of magnetic heads, i.e. to regulate average access time.

Switching to other track within one cylinder takes about one millisecond at the average. This time is made up of the insignificantly small time of switching of heads made by electronics of a hard disk drive and head positioning time. Due to manufacturing errors, the tracks in the cylinder are not located strictly under each other; there is some dispersion between them. The installation of the head precisely on the track requires reading out a certain number of servo marks, and this procedure requires some extra time. However, during one millisecond the spindle of the disk drive with the frequency of rotation of 7200 revolutions per minute manages to make almost one-eighth of the turn. Therefore, the first sector of the following track in the cylinder is shifted approximately 45 degrees against the previous one that allows avoiding "an idle" turn of a magnetic disk about the previous one.

Transition to the adjacent cylinder also requires time (standard value is 2-4 milliseconds). With this in mind, the first sector of the first track of the next cylinder is shifted to the last sector of the last track of the previous cylinder. It allows lowering losses of time for waiting for the moment when the required sector will appear under the head in the mode of a continuous reading of files. From a read speed, the linear positioning of the sectors belonging to one file is the most efficient; therefore it is necessary to do a periodical file system defragmentation to completely realize the potential of the disk drive.