MASS STORAGE
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additional memory devices called mass storage (or secondary storage)
MAGNETIC SYSTEM
The most common example in use today is the magnetic disk or hard disk drive
(HDD)
The capacity of a disk storage system depends on the number of platters used and the density in which the tracks and sectors are placed.
a thin spinning disk with magnetic coating is used to hold data
Read/write heads are placed above and/or below the disk so that as the disk spins, each head traverses a circle, called a TRACK.
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Optical Systems
Lower-capacity systems may consist of a single platter
High-capacity disk systems, capable of holding many gigabytes, or even terabytes, consist of perhaps three to six platters mounted on a common spindle.
a disk system’s performance
seek time (the time required to move the read/write heads from one track to another)
rotation delay or latency time (half the time required for the disk to make a complete rotation
access time (the sum of seek time and rotation delay)
transfer rate (the rate at which data can be transferred to or from the disk)
compact disk (CD)
disks are 12 centimeters (approximately 5 inches) in diameter and consist of reflective material covered with a clear protective coating.
CD storage systems perform best when dealing with long, continuous strings of data, as when reproducing music
was originally applied to audio recordings using a recording format known as CD-DA (compact disk-digital audio)
Traditional CDs have capacities in the range of 600 to 700MB
DVDs (Digital Versatile Disks)
provide storage capacities of several GB
are capable of storing lengthy multimedia presentations, including entire motion pictures
Blu-ray
provides over five times the capacity of a DVD
uses a laser in the blue-violet spectrum of light (instead of red)
vast amount of storage is needed to meet the demands of high definition video
Flash Drives (Flash memory devices)
SD (Secure Digital)
provide up to two GBs of storage and are packaged in a plastic rigged wafer about the size a postage stamp
Larger flash memory devices called SSDs (solid-state disks) took the place of magnetic hard disks
the use of wear-leveling techniques can reduce the impact of this by relocating frequently altered data blocks to fresh locations on the drive.
suffer from the more limited lifetime of all flash memory technologies
with capacities of hundreds of GBs, are available for general mass storage applications
bits are stored by sending electronic signals directly to the storage medium where they cause electrons to be trapped in tiny chambers of silicon dioxide, thus altering the characteristics of small electronic circuits
SDHC (High Capacity)