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Hard Drive Interview Questions

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A hard drive may fail due to multiple reasons. Some of the common causes are:

  • Head Crash: This is caused when the read / write head comes in contact with the platter. This can cause the area where it has come in contact to lose data.
  • Bad Sectors: The platter in a hard drive is split into minute areas known as sectors. Sometime certain sectors might become faulty, this does not affect the entire drive.
  • Stiction: Sometimes the head of the disk is not able to take off, it tends to stick to its position. This phenomenon is known as stiction.
  • Circuit failure: The hard disk become inoperable if any of its circuitry malfunctions.
  • Bearing and motor failure: After prolonged usage the hard disks motors or bearing tend to worn out hence not allowing normal disk performance. 

  • When a hard disk is functioning normally the read write head simply fly`s over the platter without making any contact.
  • These days to prevent the head from touching the platter the head are physically moved ( parked) to special areas also known as landing zones to prevent data loss.
  • The landing zones are usually areas where no data is stored.
  • Another approach is to leave the heads suspended in air so that they never come in contact with the platter surface.
  • In legacy devices the heads could not be automatically landed and in order to prevent them from touching the platter the user had to run programs manually to park the heads.

There are various segments for which the Hard Disks are available:

  • Desktops: They are the most common form of hard disks. The Desktop Disks can be used to store data between 60 GB to 4 TB. They generally rotate at RPM`s between 5400 and 1000. The data transfer rates for them is around 0.5 Gbit/s
  • Mobile: The hard drives belonging to this segment are smaller as compared to desktops. They have a lower capacity and are usually slower. The standard size for such segments is around 2.5 inches.
  • Enterprise: These types of hard disks are usually shared between multiple systems and data is written and read at a very fast pace. They are known to rotate at around 10k to 15k RPM, also these drives have the ability to reach sequential speeds of more than 1.6G bit/s.

  • SMART means Self Monitoring, analysis and reporting technology.
  • This is a monitoring system for hard disks used to detect and report various factors which can be used to anticipate the risk of a failure for a hard disk.
  • The failure of hard disks can be classified into predictable and unpredictable errors.
  • Predictable failures are those which are resultant from wear and tear and degradation of storage services. Unpredictable failures on the other hand occur suddenly with no warnings.
  • The SMART technology allows the user to monitor the health of the Drive and perform constant maintenance checks to prevent the chances of failures.
  • There are multiple number of self tests that can be performed by SMART such as : Short, Long/Extended, Conveyance and Selective.

  • Controllers can be considered to be an interface between the disk and the host computer.
  • These days most of the Controller circuits are placed inside the Southbridge of the motherboard.
  • The controller is also used for the following functions: Error code correction, Wear leveling. Bad Block, read scrubbing and disturb.
  • SSD are comparatively expensive as compared to Simple hard disks.
     

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