Types of storage devices

Most commonly found storages devices are  magnetic disks, magnetic tapes, automated tape libraries, CDs, and DVDs Magnetic Disks Despite of new technologies, magnetic disks have dominated nonvolatile storage since 1965. Magnetic disks play two roles in computer systems: Long-term, nonvolatile storage for files, even when no programs are running A level of the memory hierarchy below main memory used as a backing store             for virtual memory during program execution .A magnetic disk consists of a collection of platters (generally 1 to 12), rotating on a spindle at 3,600 to 15,000 revolutions per minute (RPM). Traditionally, the biggest disks have the highest performance and the smallest disks have the lowest price per disk drive. The disk surface is divided into concentric circles, designated tracks. In the past, all tracks had the same number of sectors; the outer tracks, which are longer, recorded information at a much lower density than the inner tracks.Recording more sectors on the outer tracks than on the inner tracks, called constant bit density.. This name is misleading, as the bit density is not really constant. Typically, the inner tracks are recorded at the highest density and the outer tracks at the lowest, but the outer tracks might record, say, 1.7 times more bits despite being 2.1 times longer. To read and write information into a sector, a movable arm containing a read/ write head is located over each surface . The term cylinder is used to refer to all the tracks under the arms at a given point on all surfaces. To read or write a sector, the disk controller sends a command to move the arm over the proper track. This operation is called a seek, and the time to move the arm to the desired track is called seek time. Average seek time is the subject of considerable misunderstanding. Disk manufacturers report minimum seek time, maximum seek time, and average seek time in their manuals. The first two are easy to measure, but the average was open to wide interpretation. The industry decided to calculate average seek time as the sum of the time for all possible seeks divided by the number of possible seeks. Optical Disks One challenger to magnetic disks is optical compact disks, or CDs, and its successor, called Digital Video Discs and then Digital Versatile Discs or just DVDs. Both the CD-ROM and DVD-ROM are removable and inexpensive to manufacture, but they are read-only mediums. These 4.7-inch diameter disks hold 0.65 and 4.7 GB, respectively, although some DVDs write on both sides to double their capacity. Their high capacity and low cost have led to CD-ROMs and DVDROMs replacing floppy disks as the favorite medium for distributing software and other types of computer data. They are also write-once and rewritable DVDs, called DVD-R and (alas) DVDRAM. Rewritable DVD drives cost ten times as much as DVD-ROM drives. Magnetic Tapes Magnetic tapes have been part of computer systems as long as disks because they use the similar technology as disks, and hence historically have followed the same density improvements. The inherent cost/performance difference between disks and tapes is based on their geometries: 1.      Fixed rotating platters offer random access in milliseconds, but disks have a limited storage area and the storage medium is sealed within each reader. 2.      Long strips wound on removable spools of “unlimited” length mean many tapes can be used per reader, but tapes require sequential access that can take seconds. One of the limits of tapes had been the speed at which the tapes can spin without breaking or jamming. A technology called helical scan tapes solves this problem by keeping the tape speed the same but recording the information on a diagonal to the tape with a tape reader that spins much faster than the tape is moving. This technology increases recording density by about a factor of 20 to 50. Helical scan tapes were developed for low-cost VCRs and camcorders, which brought down the cost of the tapes and readers. One drawback to tapes is that they wear out; Helical tapes last for hundreds of passes, while the traditional longitudinal tapes wear out in thousands to millions  of passes. Automated Tape Libraries Tape capacities are enhanced by inexpensive robots to automatically load and store tapes, offering a new level of storage hierarchy. These near line tapes mean access to terabytes of information in tens of seconds, without the intervention of a human operator Flash Memory Embedded devices also need nonvolatile storage, but premiums placed on space and power normally lead to the use of Flash ing. Flash memory is also used as a rewritable ROM in embedded system, typically to allow software to be upgraded without having to replace chips. Applications are typically prohibited from writing to flash memory in such circumstances. Like electrically erasable and programmable read-only memories (EEPROM). Compared to disks, Flash memories offer low power consumption (less than 50 mill watts), can be sold in small sizes, and offer read access times comparable to DRAMs.