DBMS-Storage of Databases
Databases typically store large amounts of data that must persist over long periods of time. The data is accessed and processed repeatedly during this period. This contrasts with the notion of transient data structures that persist for only a limited time during program execution. Most databases are stored permanently (or persistently) on magnetic disk secondary storage, for the following reasons:• Generally, databases are too large to fit entirely in main memory.
• The circumstances that cause permanent loss of stored data arise less frequently for disk secondary storage than for primary storage. Hence, we refer to disk—and other secondary storage devices—as nonvolatile storage, whereas main memory is often called volatile storage.
• The cost of storage per unit of data is an order of magnitude less for disk than for primary storage.
Some of the newer technologies—such as optical disks, DVDs, and tape jukeboxes—are likely to provide viable alternatives to the use of magnetic disks. For now, however, it is important to study and understand the properties and characteristics of magnetic disks and the way data files can be organized on disk in order to design effective databases with acceptable performance.
Magnetic tapes are frequently used as a storage medium for backing up the database because storage on tape costs even less than storage on disk. However, access to data on tape is quite slow. Data stored on tapes is off-line; that is, some intervention by an operator—or an automatic loading device—to load a tape is needed before this data becomes available. In contrast, disks are on-line devices that can be accessed directly at any time.
The techniques used to store large amounts of structured data on disk are important for database designers, the DBA, and implementers of a DBMS. Database designers and the DBA must know the advantages and disadvantages of each storage technique when they design, implement, and operate a database on a specific DBMS. Usually, the DBMS has several options available for organizing the data, and the process of physical database design involves choosing from among the options the particular data organization techniques that best suit the given application requirements. DBMS system implementers must study data organization techniques so that they can implement them efficiently and thus provide the DBA and users of the DBMS with sufficient options.
Typical database applications need only a small portion of the database at a time for processing. Whenever a certain portion of the data is needed, it must be located on disk, copied to main memory for processing, and then rewritten to the disk if the data is changed. The data stored on disk is organized as files of records. Each record is a collection of data values that can be interpreted as facts about entities, their attributes, and their relationships. Records should be stored on disk in a manner that makes it possible to locate them efficiently whenever they are needed.
