SECONDARY STORAGE Secondary storage provides cheap, non-volatile high capacity storage. There Are basically two types of secondary storage: * Magnetic devices * Optical devices MAGNETIC DEVICES Magnetic Disc The Magnetic Disk is Flat, circular platter with metallic coating that is rotated beneath read/write heads. It is a direct access device; read/write head can be moved to any location on the platter. Magnetic disks provide storage for large amounts of data and instructions that can be rapidly accessed. For data recording, the surface of a disk is divided into a number of invisible concentric circles called tracks.
The tracks are numbered consecutively from outmost to innermost starting from zero. The number of tracks varies greatly between disks, from as few as 40 on some small, low capacity disks to several thousand on large, high capacity disks. Each track is further subdivided into sectors. For this, in addition to the concentric circles, the disc surface is also divided into invisible pie-shapes segments. Thus if there are eight such pie shaped segments, each track will get divided into eight parts, and each of these eight portions of a track is called a sector.
Storage capacity depends on • Number of recording surfaces • Numbers of tracks per surface • Number of sectors per track • Number of byes per sector Magnetic Tape A magnetically coated strip of plastic on which data can be encoded is called a magnetic tape. Tapes for computers are similar to tapes used to store music. Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have large storage capacities, ranging from a few hundred kilobytes to several gigabytes. Accessing data on tapes, however, is much slower than accessing data on disks.
Tapes are sequential-access media, which means that to get to a particular point on the tape, the tape must go through all the preceding points. In contrast, disks are random-access media because a disk drive can access any point at random without passing through intervening points. Because tapes are so slow, they are generally used only for long term storage and backup. Data to be used regularly is almost always kept on a disk. Tapes are also used for transporting large amounts of data. Tapes come in a variety of sizes and formats. Floppies A soft magnetic disks called a floppy.
Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable. Floppies come in three basic sizes: * 8-inch: The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop computer does not use the 8-inch floppy disk. 5? -inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1. 2MB (megabytes) of data. The most common sizes are 360K and 1. 2MB. * 3? -inch: The most common sizes for PCs are 720K (double-density) and 1. 44MB (high-density). Macintoshes support disks of 400K, 800K, and 1. 2MB. OPTICAL DEVICES Optical storage devices store bit values as variations in light reflection. They have higher area density & longer data life than magnetic storage.
They are also standardized and relatively inexpensive. Their uses are: read-only storage with low performance requirements, applications with high capacity requirements & where portability in a standardized format is needed. Another advantage of optical storage is that the medium itself is less susceptible to contamination or deterioration. Optical drives are also less fragile, and the disks themselves may easily be loaded and removed. In addition, optical disks can store much more information, both on a routine basis and also when combined into storage systems. Their Types: CD-ROM (read only) * DVD (Digital Video Disk) * Blu-ray disc Compact disk read-only memory (CD-ROM) It is portable disk having data storage capacity between 650-700 MB. It can hold large amount of information such as music, full-motion videos, and text etc. It contains digital information that can be read, but cannot be rewritten. Separate drives exist for reading and writing CDs. Since it is a very reliable storage media, it is very often used as a medium for distributing large amount of information to large number of users. In fact today most of the software is distributed through CDs.
Compact disk, rewritable (CD-RW) adds rewritability to the recordable compact disk market, which previously had offered only write-once CD-ROM technology. DVD Digital Versatile Disk (DVD) is similar to a CD but has larger storage capacity and enormous clarity. Depending upon the disk type it can store several Gigabytes of data (as opposed to around 650MB of a CD). DVDs are primarily used to store music or movies and can be played back on your television or the computer too. They are not rewritable media. * Over 4 GB storage (varies with format) DVD- ROM (read only) * Many recordable formats (e. g. , DVD-R, DVD-RW; .. ) * Are more highly compact than a CD. * Special laser is needed to read them. Blu-ray Technology The name is derived from the blue-violet laser used to read and write data. It was developed by the Blu-ray Disc Association with more than 180 members. Some companies with the technology are Dell, Sony, LG. The Data capacity is very large because Blu-ray uses a blue laser(405 nanometers) instead of a red laser(650 nanometers) this allows the data tracks on the disc to be very compact.
This allows for more than twice as small pits as on a DVD. Because of the greatly compact data Bluray can hold almost 5 times more data than a single layer DVD. close to 25 GB. Just like a DVD ,Blu-ray can also be recorded in Dual-Layer format. This allows the disk to hold up to 50 GB. The Variations in the formats are as follows: • BD-ROM (read-only) – for pre-recorded content • BD-R (recordable) – for PC data storage • BD-RW (rewritable) – for PC data storage • BD-RE (rewritable) – for HDTV recording
