SEB 070021 - TUTORIAL 5

MAGNETIC DISK




IBM introduced the first magnetic disk, the RAMAC, in 1955; it held 5 megabytes and rented for $3,200 per month. Magnetic disks are platters coated with iron oxide, like tape and drums. An arm with a tiny wire coil, the read/write (R/W) head, moves radially over the disk, which is divided into concentric tracks composed of small arcs, or sectors, of data.

DESCRIPTION

MEDIA MECHANICS – Multiple fixed disk
DRIVE MECHANIS – Excellent
POSITION ERROR SIGNALS – Mediocre SNR / Multiplexed with Data
SAMPLE RATE – Low/ Medium
VERTICAL POSITION – Air Bearings : Near field, No focus loop, Multiple small
Heads
TRACKING LOOP – Single medium-high frequency
SPINDLE LOOP – Low frequency
SPINDLE MODE – Constant Angular Velocity (CAV)
TRACKS – Circular
APPLICATIONS – Mostly random access

ADVANTAGES

~ Hard disk space is relatively cheap, as low as 13p per GB
~ Hard disks store data without the need for a constant electricity supply
~ Hard disks allow data to be stored in one place which is more convenient than using DVDs for example.
~ Hard drives have a read/write speed in comparison to optical media (cds) although very low in comparison to RAM

OPTICAL DISK

An optical disk is a compact disk or CD. The formatting of the optical disk will dictate whether it is a DVD, CD, read-only or rewritable. The optical disk is so named because its technology is based on light. As the disk spins, a laser beam follows a spiraling trail of pits and lands in the plastic material of the disk. The pits reflect light differently than the lands, while a device translates the reflective difference to bits of "on/off" or 1 and 0. The bits form bytes that carry the digital code of the data stored on the optical disk.

DESCRIPTION

MEDIA MECHANICS – Single removable disk
DRIVE MECHANIS – Mediocre
POSITION ERROR SIGNALS – Excellent SNR / continuously available
SAMPLE RATE – High
VERTICAL POSITION – No air bearings : far field, focus loop, single large
heads
TRACKING LOOP – Coarse (low frequency) and fine ( high frequency)
SPINDLE LOOP – Low frequency
SPINDLE MODE – Constant Angular Velocity (CAV) and Constant Linear
Velocity (CLV)
TRACKS – Predominantly spiral, some circular
APPLICATIONS – Mostly streaming files

ADVANTAGES

~ Compact, lightweight, durable and digital,
~ Mass storage capacity (on the order of gigabytes) . A double-layered and double-sided DVD optical disk holds up to 15.9 gigabytes (GB) of data. The optical disk also provides a minimum of 650 megabytes (MB) of data storage.
~ Mountable/uncountable storage units
~ Long media life
~ High data stability

FLASH MEMORY

DESCRIPTION

Flash memory refers to a particular type of EEPROM, or Electronically Erasable Programmable Read Only Memory. It is a memory chip that maintains stored information without requiring a power source. Flash memory can erase its data in entire blocks, making it a preferable technology for applications that require frequent updating of large amounts of data as in the case of a memory stick. Inside the flash chip, information is stored in cells. A floating gate protects the data written in each cell. Tunneling electrons pass through a low conductive material to change the electronic charge of the gate in "a flash," clearing the cell of its contents so that it can be rewritten. This is how flash memory gets its name.

ADVANTAGES

Flash memory used as a hard drive has many advantages over a traditional hard drive. It's nonvolatile or solid state, meaning there are no moving parts. It's also silent, much smaller than a traditional hard drive, and highly portable with a much faster access time. That said, the price of flash memory continues to drop as capacity continues to rise, making it a prize candidate for an ever-broadening set of applications.


MAGNETO OPTICAL DISK

DESCRIPTION

Magneto-optical (MO) drives are generally understood to be the one of the most popular methods of data storage for many businesses. Using cartridges that are very similar in appearance to the older 3.5 inch diskettes that were popular in the 1990’s, the magneto-optical drive provides a means of storing huge amounts of data in a very small storage unit. Unlike the older diskettes, the cartridges that fit into an MO drive typically hold up to several gigabytes of information with no problem.
Part of what helps to make the magneto-optical drive so efficient is that configuration of the device. MO drives make use of a dual method to read and write data to the cartridge. Both lasers and a head configured with read and write ability scan and save the data onto the cartridge. The amount of time required to scan and process information with a magneto-optical drive is actually much quicker than some older methods of backing up files, such as tape backups or even the use of CDR backups. Because the capacity on a single cartridge is so great, there is no need to use multiple units to capture and store the data, even when copying huge data files.

ADVANTAGES

Another advantage to the cartridges used with a magneto-optical drive is the fact that they can be erased and reused multiple times, just like most other types of storage devices. For companies that choose to copy key data files on a daily basis and keep them in storage for only a limited amount of time, this means the older cartridges can be wiped and used again, cutting down on expenses associated with archiving important data.