DBMS-Magnetic Tape Storage Devices
Disks are random access secondary storage devices, because an arbitrary disk block may be accessed "at random" once we specify its address. Magnetic tapes are sequential access devices; to access the nth block on tape, we must first scan over the preceding n - 1 blocks. Data is stored on reels of high-capacity magnetic tape, somewhat similar to audio or video tapes. A tape drive is required to read the data from or to write the data to a tape reel. Usually, each group of bits that forms a byte is stored across the tape, and the bytes themselves are stored consecutively on the tape.A read/write head is used to read or write data on tape. Data records on tape are also stored in blocks—although the blocks may be substantially larger than those for disks, and interblock gaps are also quite large. With typical tape densities of 1600 to 6250 bytes per inch, a typical interblock gap of 0.6 inches corresponds to 960 to 3750 bytes of wasted storage space. For better space utilization it is customary to group many records together in one block.
The main characteristic of a tape is its requirement that we access the data blocks in sequential order. To get to a block in the middle of a reel of tape, the tape is mounted and then scanned until the required block gets under the read/write head. For this reason, tape access can be slow and tapes are not used to store on-line data, except for some specialized applications. However, tapes serve a very important function—that of backing up the database. One reason for backup is to keep copies of disk files in case the data is lost because of a disk crash, which can happen if the disk read/write head touches the disk surface because of mechanical malfunction. For this reason, disk files are copied periodically to tape. Tapes can also be used to store excessively large database files. Finally, database files that are seldom used or outdated but are required for historical record keeping can be archived on tape. Recently, smaller 8-mm magnetic tapes (similar to those used in camcorders) that can store up to 50 Gbytes, as well as 4-mm helical scan data cartridges and CD-ROMs (compact disks–read only memory) have become popular media for backing up data files from workstations and personal computers. They are also used for storing images and system libraries. In the next Section we review the recent development in disk storage technology called RAID.
