【正文】 however, unlike oldfashioned records, the track on a CD spirals from the inside out .This track is divided into units called sectors, each with its own identifying markings and a capacity of 2KB of data, which equates to 1／ 75 of a second of music in the case of audio recording . Information is stored on a CD at a uniform linear density over the entire spiraled track, which means that more information is stored in a loop around the outer portion of the spiral than in a loop around the inner portion. In turn, more sectors will be read in a single revolution of the disk when the laser beam is scanning the outer portion of the spiraled track than when the beam is scanning the inner portion. Thus, to obtain a uniform rate of data transfer, CDDA players are designed to vary the rotation speed depending on the location of the laser beam. However, most CD drives used for puter data storage spin at a faster, constant speed and thus must acmodate variations in data transfer rates. As a consequence of such design decision, CD storage systems perform best when dealing with long, continuous string of data, as when reproducing music. In contrast, when an application requires access to items of data in a random manner, the approach used in magic disk storage (individual, concentric tracks) outperforms the spiral approach used in CDs. Traditional CDs have capacities in the range of 600 to 700MB. However. Newer DVD (Digital Versatile Disks),which are constructed from multiple, semitransparent layers that can be distinguished by a precisely focused laser, provide storage capacities on order of several GB. Such disks are capable of storing lengthy multimedia presentations, including entire motion pictures. Magic Tape An older form of mass storage device uses magic tape. Here, information is recorded on the magic coating of a thin plastic tape that is wound on a reel for storage. To access the data, the tape is mounted in a device called a tape drive that typically can read, write, and rewind the rape under control of the puter. Tape drives range in size from small cartridge units, called streaming tape units, which units. Although the capacity of these devices depends on the format used, most can hold many gigabytes. Modern streaming tape systems divide a tape into segments, each of which is magically marked by a formatting process similar to that of disk storage devices. Each of these segments contains several tracks that run parallel to one another lengthwise on the tape. There tracks can be accessed independently, meaning that the tape ultimately consists of numerous individual stings of bits in a manner similar to the sectors on a disk. A major disadvantage of magic tape technology is that moving between different positions on a tape can be very timeconsuming owing to the significant amount of tape that must be moved between the reels, Thus tape systems have much longer data access times than magic disk systems in which different sectors can be accessed by short movements of the read/write head. In turn, tape systems are not popular for online data storage Instead, magic tape technology is used in offline archival data storage applications where its high capacity, reliability, and cost efficiency are beneficial, although advances in alternative technologies such as DVD are rapidly challenging this last vestige of magic tape. File Storage and Retrieval Information is stored in mass storage systems in large units called files. A typical file may consist of a plete text document, a photograph, a program, or a collection of data about the employees in a pany. The physical properties of mass storag