Physical Storage Devices

Physical Storage devices

In today's computer systems there are several types of data storage exist. These storage media are classified by the speed with which data can be accessed, by the cost per unit of data to by medium, and by the medium's reliability. These media devices are :

Cache : The Cache is the fastest and most costly form of storage. Cache memory is small, its use is managed by the computer system hardware. This memory is typically integrated directly with the CPU chip or placed on a separate chip that has a separate bus interconnect with the CPU.

Main memory : The Storage medium used for data that are available to be operated on is main memory. The general-purpose machine instructions operate on main memory. Although main memory may contain many megabytes of data, or even gigabytes of data in large server systems. 

Flash memory: Flash memory is an Electronic, solid-state and non-volatile computer storage medium that can be electrically erased and reprogrammed. It is also known as EEPROM(Electrically erasable programmable read-only memory), flash memory differs from main memory in that data survive power failure. Reading data from flash memory takes less than 100 nanoseconds (a nanosecond is 1/1000 of a microsecond), which is roughly as fast as reading data from main memory. However, writing data to flash memory is more complicated - data can be written once, which takes about 4 to 10 microseconds, but cannot be overwritten directly. To overwrite memory that has been written already, we have to erase an entire bank of memory at once, it is then ready to be written again. A drawback of flash memory is that it can support only a limited number of erase cycles, ranging from 10000 to 1 million. Flash memory has found popularity as a replacement for magnetic disks for storing small volumes of data (5 to 10 megabytes) in low-cost computer systems, such as computer systems that are embedded in other devices, in hand-held computers, and in other digital electronic devices such as digital cameras.

Magnetic-disk storage: A magnetic disk is a storage device that uses a magnetization process to write, rewrite and access data. It is covered with a magnetic coating and stores data in the form of tracks, spots and sectors. Hard disks, zip disks and floppy disks are common examples of magnetic disks. A magnetic disk primarily consists of a rotating magnetic surface and a mechanical arm that circulates over it. The mechanical arm is used to read from and write data to the disk. The data on a magnetic disk are read and written using a magnetization process. Typically, a magnetic disk is the primary storage disk in a computer. Data are organized on the disk in the form of tracks and sectors, where tracks are the circular divisions of the disk. Tracks are further divided into sectors that contain blocks of data. All read and write operations on the magnetic disk are performed on the sectors.

Optical Storage: The most popular forms of optical storage are the compact disk (CD), which can hold about 700 megabytes of data, and the digital video disk(DVD) or digital versatile disc which can hold 4.7 or 8.5 gigabytes of data per side of the disk (or up to 17 gigabytes on a two-sided disk). Data are stored optically on a disk, and are read by a laser. The optical disks used in read-only compact disks (CD-ROM) or read-only digital video disk (DVD-ROM) cannot be written, but are supplied with data prerecorded. There are “record-once” versions of compact disk (called CD-R ) and digital video disk (called DVD-R ), which can be written only once; such disks are also called write-once, read-many ( WORM ) disks. There are also “multiple-write” versions of compact disk (called CD-RW ) and digital video disk ( DVD-RW and DVD-RAM ), which can be written multiple times. Recordable compact disks are magnetic–optical storage devices that use optical means to read magnet-ically encoded data. Such disks are useful for archival storage of data as well as distribution of data.

Tape Storage : Tape storage is used primarily for backup and archival data. Although magnetic tape is much cheaper than disks, access to data is much slower, because the tape must be accessed sequentially from the beginning. For this reason, tape storage is referred to as sequential-access storage.  In contrast, disk storage is referred to as direct-access storage because it is possible to read data from any location on disk.

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