Transcript HARD DRIVES
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? Structure of hard drives Or Not 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.