Rise of the Disk

Printers  > General > Disk Capacity

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Disks of various kinds seem to beat paper as a distribution and storage medium many times over.
 

CDs can hold hundreds of books (books vary from 1 to 20 megabytes so a 700mb CD hold at least 35.

DVDs hold about 10 to 20 times as much as a CD - with structuring and compression still more.

Hard Disks can hold hundreds of times more than a DVD. Where CDs and DVDs are just plastic disks with a pattern printed in aluminium a hard disk is a complex physical assembly.

Home computer users typically keep CDs and DVDs on a shelf and load tham as needed. Various bits of software have to engage, so accessing a DVD is often slower than picking up a book.

Corporate users can load information into Multimedia jukeboxes ornetworked disk arrays. A networked disk array can fairly easily exceed the storage capapcity of the largest libraries and have response times of under a second.

It rather looks as though paper is obsolescent. Reports, magazines and books would be distributed faster, cheaper and very much more conveniently on disk. Internet sharing using peer to peer mechanisms like bittorrent can create robust public and private storage systems beyond the capabilities of any paper library. The new generation of hard-disks can hold

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The idea of the computer as a giant brain holding every fact known to humanity goes way back into science fiction - the problem being that nobody had any real idea how that might be done. Sci-Fi movies tend to show gleaming tape drives with the storage-bin action that became popular in the 1950s - they look good but don't actually work terribly well.

Disk drives are the answer - at least for now. Recent disk drives are uninteresting from the outside; just a small metal box with a connector. Inside is a shiny disk spins under a radial arm carrying ead-write heads. Disk drives store information magnetically. A disk spins several hundred times per second and the read/write head forms tracks of little magnetised areas on the disk. The magnetised patterns persist for years - or until they are replaced by something new. When the data is wanted the machine positions the head to find the pattern again.

Data bits in current drives are about 50 nanometers across - 20,000 to the millimetre. Depending on how bit coding is done that might work out as 100 million bits per square millimetre of surface and since there are about 5,000 square millimetres of surface and two surfaces that gives about a million million bits - a terabit, which in turn is 120 gigabytes. Things are not quite so simple because modern disks tend to use run-length coding and Reed Solomon correction codes so what goes in at the interface and what gets written on the surface can be rather different.

The inner disk shines like a mirror because its optically perfect - the data bits are smaller than a wavelength of light so if there were any roughness that could be seen it would be disruptive. Looking inside a drive is the end of it's life because a single particle of dust will wreck it.

Disk drives are critical parts of most computers. To behave at all intelligently and deliver a wide range of information a computer requires programs and data which can come from a disk or a network. Data often does come from a network - but the network is probably getting it from disk somewhere. Almost every machine from an ipod to a supercomputer has a disk. Disk capacity sets what the computer can do - so larger computers tend to have racks full of them.

Manufacturers have several options to make bigger disk drives.

The capacity of any given disk technology can be doubled by using two or more disk platters and double the number of heads - with an increase in cost. Common disks at present hold a couple of hundred gigabytes. The largest ordinary disk drives hold about a terabyte. Capacities usually double every year or so.

Capacity increases mainly come from technological advance - making the data bits smaller. Early disk drives had mechanisms ather like those in magnetic tape drives - big clunky motors and ead-write heads. Today's disk heads, platter materials, motors and electronics look very different - although the basic idea is the same.

Disks are used because they turn out to have the best geometry for making very small magnetic marks. The disk spins continually when it is in use and a read-write head moves radially in and out carrying the head - which these days is a little sliver of ceramic and silicon. Disk action can be simple and stable and this is part of it's beauty. Cards and cylinders have been tried but the engineering tends to be more complicated and finding marks less reliable. Tape is used for some kinds of backup - but doesn't pack data quite so well.

Inside a disk is a beautiful bit of precision engineering. Disk heads and platters could be like jewellry, made in one-off for very special clients. The economics of disk manufacturing don't work like that. The best way to make disks is on production lines that make millions of identical objects that can be sold in large quantity. Whenever it has seemed that semiconductor flash memories and networks would finally allow computers that didn't need disks manufacturers have come up with a still bigger device that can deliver a new trick. The latest generation of disk drives are going into audio visual gear - not just ipods but personal video ecorders and video cameras as well. It takes gigabytes of capacity to record eal-time multi-megapixel video but recent disk drives can deliver that.

Making disks that can hold the amount of data in a couple of hundred feature-films has an interesting impact on many less demanding tasks like holding business records. Tasks which used to equire seried ranks of disks can be done by something pocket sized that anyone can afford. A modern 200 gigabyte disk drive can in theory hold the equivalent of about 20 million pieces of paper - record keeping equivalent to a hospital or bank. Of course a hospital will now want to store X-rays and video training courses so an organisation will still tend to have racks of disks to accomodate new demands.

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From their invention in the mid 1950s it has always been possible to make big disk storage systems - for a government intelligence agency with a huge budget to throw at the problem.

Until the mid 1980s most people would have expected that even if computer use became very common the future lay with big central storage systems. Centralised systems may still be the future but the emergence of high-capacity, low-cost disk systems makes that disputable.

High capacity, low cost disk drives make it possible for computers to hold vast ammounts of information - and probably not just on central servers.

Personal computers have become popular - sufficiently so that most workplaces have one for every administrative desk and half the homes in the UK have one. Personal computer ownership will continue to expand - (games and markets in the developing world mean demand goes on expanding). In all probability, however, not everyone will ever want  a more exciting area for manufacturers is
 
 

Flash memory

The price of data storage goes down rapidly but at present (early 2006) the price of a computer textbook and a 120 gigabyte disk drive are roughly comparable - about £40 in the UK. Disks can contain any digitisable information so that works out at the equivalent of about 200 symphonies in high-fi sound, 120 TV quality movies or a medium sized library of about 25,000 books.