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Tuesday, May 19, 2009

Hard disks: Technology & Failure

How and why a hard drive works leads us to understand why they and how hard drives fail.
Mechanisms implemented to avoid loosing data caused by certain hard disk failures:
  1. S.M.A.R.T. This system was notifies the user when the drive may be failing. Several components such as drive speed, track to track time, head placement and many other tests are performed on the drives daily. These tests are used in a comparison over time and can show a pattern of device failure. During the degradation cycle there is a manufacturer assigned percentage used that will alert the user of impending data loss. For instance, if a drive has been spinning at 7200 RPM for several months, then over the last several weeks the RPMs drop to 7000 RPM, this may be flagged as an impending problem and the user will be notified that possible data loss may occur in the near future. It is up to the user to backup the data and replace the failing hard disk drive or prepare for data recovery.
  2. Sector Map. The sector map consists of two lists. The primary list, which is set at factory, maps all bad sectors out. These bad sectors are found and mapped before the drive is sold. In other words, hard drives are expected to have bad sectors. There is no such thing as a perfect hard drive. A hard disk drive would cost several thousand dollars to the consumer if manufacturers created the means to make a perfect hard drive. In order to keep costs down, drive remapping is used. Now, once the list of all the bad sectors are made at the factory the drive is then shipped and sold. As the drive performs its day to day functions more bad sectors will appear. It is inevitable. These bad sectors are then mapped to another list. Both of the bad sector lists are mapped to a reserved area of the hard drive. There are only so many reserved area sectors for the firmware to remap to. Once these sectors are exhausted the firmware cannot remap and the bad sectors become part of your live data.

Signs that a drive has used its reserved sector area are:
slow reads, slow boot sequence, operating system may seem to lock, may have to reboot several times before the operating system comes up.
A chronic bad sector problem will eventually cause file loss, and ultimately the drive will not be able to identify itself to the BIOS and your data is either lost, or you will need hard drive recovery services.

The hard disk drive has one or more platters that are made of glass or aluminum coated with magnetic iron oxide particles. The platters spin continuously as the heads travel in and out along a seek pattern. A typical hard drive contains several of these 3.5-inch platters, which can contain tens of billions of individual bits.

The higher the areal density of the hard disk’s platters, the more bits that can be packed into each square inch of platter real estate. A platter is segregated into tens of thousands of concentric tracks.
Because tons of information can be stored in one track, the tracks are broken down into smaller units called sectors.
Each sector can hold about 512 bytes of data. Disk platters are mounted in a stacked formation on a spindle A spindle motor turns the platters at very high speed, typically between 5,400 and 7,200 rotations per minute, but as fast as 15,000 rotations per minute(RPM). The platters spin so that the appropriate sector or sectors containing the data can be positioned underneath one of the drive’s reading heads. There’s one head per platter, and all the heads move in unison.

Each head in the hard drive is mounted onto a slider, which is mounted onto an arm. A mechanical device called an actuator controls each hard drive arm. The actuator moves the arm to the correct position on the spinning platter, which puts the head in the correct position. The reading head ( reading and writing heads are separate) floats about 2/1,000,000 of an inch above the disk surface. As it passes over the appropriate disk sectors, it interprets the magnetic pulses and converts them to electrical pulses that can be interpreted as 1s and 0s.

Although the head may look large, actually the sensitive part of the head is defined by micro lithographic methods so that the actual exposed portion of the head,which can either read or write the information on the disk,is very small-comparable to the dimensions of features on a microchip! As the manufacturing technology has improved over the years they have been able to shrink this area down to smaller and smaller sizes, and that is how the hard disk memories have risen to such large figures now. In addition they have increased the density of magnetic particles on the disk which makes the storage capacity larger.
For example in 1991 the density of storage memory was about 0.l3 Gbits/square inch.
In 1998 it was up to around 4 Gbits/square inch (30X!).
At the same time the lithography limit of the sensitive head area was about 4.5 uMeters in 1991, and in 1998 was down to 0.5 uMeters, reduced by a factor of 9. So that is how the storage density of the disks is now up to 200 GB or higher.

Any deviation in the platter or head can cause the hard drive to start clicking. If it starts clicking you will need hard drive recovery.

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