HARD DRIVES by WaKUy9W

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									HARD DRIVES
   HARD DRIVES


1. Fundamental Hard Drives


 2. Advanced Hard Drives


                       By Joe Watson
     1. FUNDAMENTAL HARD
            DRIVES

   Hard Drives or Hard Disks are the most
    common form of backing storage in modern
    personal computers

   Do you want to know MORE?    Or Not


    Structure of hard drives
        Hard drives are a form of magnetic storage media.
Inside the hard outer shell there is a series of hard, shiny
disks called ‘platters’.
        The data is stored on these platters in blocks that
can be magnetised and demagnetised.
        The data is written to and read from the disk by a
series of heads that touch the platters.
        The platters rotate on a drum that spins very
quickly.
        All of this is housed inside a capsule that protects
the interior from anything that may affect the performance
of the disk such as dust, water of smoke particles.
A break up of the internal workings:
And what one actually looks like:
 Hard disks have revolutionised the
  computer industry since their introduction
  in the early 1980’s.
 All aspects of them have made the hard disk
  a very popular choice of backing storage for
  comuter manufacturers.
                1. Size
                2. Speed
                3. Cost
1. Size

   In the early 1980’s a typical hard drive for a
  microcomputer would have had the capacity of
                 around 10Mbytes!
   Nowadays, a typical PC hard drive ranges
 between 5 and 50 GBytes with some ‘monster’
 hard drives up to 180 Gbytes (the largest at the
                    moment).

                                             MORE
 Physical Size
       The physical size of hard drives continues to
get smaller (at the same time as capacity continues to
get larger). This is simply because the amount of data
that engineers cna fit onto a square inch of magnetic
media is doubling every six months.
       The latest SIZE revelation is the new miniature hard
drives which are literally no larger than a credit card (and only
about twice as thick!!)
2. Speed

       The speed of the hard disk unit has also
increased greatly over the last decade, from around
10 KBytes per second to a massive 40Mbytes per
second and is still rising!!!!!!!.
       The internal drum of the hard drive (the thing
that the platters spin on) operate very, very quickly.
The fastest ones can spin at around 10,000 rpm.
3. Cost

       Basically, the cost of hard drives increases
with capacity, speed and performance. The cost of a
hard drive is often measured in ‘£ per Mbyte’.
      10 years ago the cost per Mb was in the area
of £120 but now the cost per Mb is usually below
40pence
        The most expensive hard drives can be well
over £1000 but some great deals are to be had
through stockists that can get you an adequate hard
drive, very cheaply (usually under £100).
     Advanced Hard Drives

Unfortunately, that has barely touched on the
surface and now that you know the basics we
       can move on to the tricky stuff.
   How is data arranged on the disk?

         The disk is split up into tracks and segments. The
    tracks are crcles, radiating out from the middle and the
    segments split the disk up in lines heading out from the
    middle.

         A disk can have several thousand tracks and segments.
    This forms individual ‘sectors’ on the disk and it is in these
    sectors that the data is addressed and stored.
         The process of setting up the disk so that it is ready to
    be used is called ‘formatting’
                                                        DIAGRAM
• While the disk is whizzing around it is obvious that
some form of identification will be needed to to istruct
the computer where the beginning of a track or cylinder
is. For example, you need to know where sector zero
begins on each track.
• Sometimes a small hole in the disk is used in
conjunction with an optical detection system and a light
shining through the hole, to mark the start of the sector.
• Using physical marking such as this is called a Hard
Sector
• Using an electronic ID to mark sectors on the disk is
called Soft Sectoring.
Clusters
   These are simply the minimum amount of space
    allocated by the operating system when it stores a
    file onto the hard disk.
    – If there are 128 sectors per cluster on a large disk and
      you only want to store a file of size 1 byte, that would
      leave 65,535 bytes of storage space unused.
    – To combat this it is often a good idea to partition large
      drives so that they appear to the computer as a number
      of smaller drives.
 Disk Caching
• Providing some Dedicated RAM for disk caching
  can significantly improve the apparent disk
  performance. There are 2 types of disk caching
  systems.
      1. Least recently used
      2. Least frequently used

• Both these systems are self descriptive in their
  method of choosing data to send to RAM and to
  delete from RAM
• The advantage of Disk Caching is simply that read
  and write operations to and from backing storage
  can take place at RAM access speeds.
Disk array systems and RAID
 Unfornuately, hard disk systems will eventually
  crash. Normally this would mean losing an awful
  lot of what is probably very important data which,
  even in a best case scenario, is an inconvenience.
 RAID (Redundant Array of Inexpensive Drives) is simply a
  set of hard drives that can be turned on for use
  when the primary disks fail.
 Disk arrays are a set drives that are basically a
  mirror image of the primary ones and are kept up
  to date all the time. A good software package with
  this will prevent a disk crash being as bad as it
  could be.
Tetrabyte Disk Farms
   Hard disks are not only used in personal computers. They
    are used in very large companies that have to store an
    awful lot of data.
   Because of the large volume of data it is neccessary to
    have a large number of primary and backup disks all linked
    together.
   Companies can make systems like this, with hundreds and
    often thousands of Gbytes worth of hard disk space.
   An example of this is the Fujitsu F6497KC that can store
    about 715 Gbytes of data and incorporates RAID 5
    technology which allows the system to carry on during any
    from of disk crash.
        By now, yo’re probably feeling a bit like this.
Drive Technologies
   There are a few disk drive technologies in current
    operation
     – IDE Integrated Drive Electronics
     – EIDE Enhanced Integrated Drive Electronics
     These are usually at the smaller and cheaper end of the
       market.
     The much faster and more expensive are:
     – SCSI Small Computer Systems Interface
     This comes in several versions (1,2,3 and an Ultra SCSI
       high-speed mode.
     Very rescent technology such as Firewire are still in the
       development pipeline but provide a lot of promise for
       the future of technological advances in Hard Drives.

								
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