Wednesday, October 8, 2014

What are the performance measures of hard disk drives?

Performance measures of hard disks / Important measures used for finding the hard disk drive performance / The major performance measures of a HDD


What are the performance measures of hard disks?

Function of a hard disk

First, let us see the function and internals of a hard disk drive. See the figure given below for the essential things inside the hard disk arrangement for our discussion.
Figure 1 - internal component of a hard disk drive
Platter - flat circular disk (like CDs or DVDs) that hold data
Track - the logical division of a side of a platter 
Read-write head - a small part of hard disk that move above the disk platter to read or write data from or to disk. [read-write head transform the platter's magnetic field into electrical current - reading, read-write head transform the electrical current into magnetic field - writing]
Spindle - fixed setup which rotates the platters

How does a hard disk drive function?

If a request is initiated by the Operating System to read a file, the information is passed on to the Disk-controller, and the disk-controller initiates necessary actions.

  1. Read-write head is positioned over the right track where the required data might be stored.
  2. Platters are rotated to bring the right sector under the read-write head.
  3. The data transfer begins at this point.

Watch the video to see the movements of the internal components. [video is taken from].

Various performance measures of hard disk

They are,

  • Disk capacity,
  • Access time,
  • Data-transfer rate
  • Reliability,
  • Power consumption and Shock resistance.

Disk Capacity – the amount of data which can be stored (measured in terms of GBs / TBs today).
Access time – the time consumed between the request initiation and the beginning of data transfer.
For example, if you would like to open a file, you need to click/double click the file. That moment, the request is generated. This is the request initiation time. To access the data on a particular sector, the disk arm is positioned the read/write head over the correct track of the required sector. Then the disk platter is rotated to position the correct sector under the read-write head. At this moment the transfer begins. This is the data transfer beginning time. The actual time consumed between these two times is called the Access time.
The key components that are considered in calculating the Access time are;

  • Seek time – the time for repositioning the arm so that the read-write head can be positioned over the correct track. [Average seek time for Seagate hard disk drive Barracuda 7200.9 is 11 ms approximately]

  • Rotational latency time – the time spent for the required sector to appear under the read-write head. [Typical rotational latency for hard disk drive with the speed of 7200 rpm is 4.17 ms approximately]

Access time = Seek time + Rotational latency time
Data transfer rate – the rate (speed) at which data can be read from / written to a disk. [Maximum external transfer rate for Seagate hard disk drive Barracuda 7200.9 is 300 MB/s approximately].

Data transfer rate = minimum (the speed at which the data can be moved between disk surface and disk controller, the speed at which the data can be moved between disk controller and the host system)
For example,
Data transfer rate = minimum (100 MB/s, 150 MB/s) = 100 MB/s.
Reliability – How far a disk is reliable, ie., free from failure is one of the major measures. Mean-Time-To-Failure (MTTF) is the measure used to calculate the reliability of the disk. [Typical MTTF value ranges from 30,000 hours to 1,200,000 hours today]
Power consumption and Shock resistance– they are the other issues which are very much analyzed in today’s scenario for the cases mobile devices such as laptops. To get rid of waste heat pollution, these measures should be considered for even desktop computers also.

External link -
Hard disk drive performance characteristics 

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