The primary goal of physical database design is data processing efficiency. Today, with ever-decreasing costs of computer technology per unit of measure, it is typically important that the physical database design must minimize the time required by users to interact with the information system. During physical database design, the database designer translates the logical description of data into the technical specifications for storing and retrieving data. A physical file is a named portion of secondary memory allocated for the purpose of storing physical records. Data within a physical file are organized through a combination of sequential storage and pointers. A file organization arranges the records of a file on a secondary storage device. The three major categories of file organizations are sequential file organization, index file organization, and hash file organization. In sequential file organization, records are stored in a sequence to a primary key value. In index file organization, records are stored sequentially or nonsequentially and an index is used to keep track of where the records are stored. In hash file organization, the address of each record is determined using an algorithm that converts a primary key value into a record address. In this chapter the different types of file organization are explained through illustrative examples. File access efficiency and file reliability can be enhanced by the use of a RAID, which allows blocks of data from one or several programs to be read and written in parallel to different disks, thus reducing the input/output delays with traditional sequential I/O operations on a single disk drive. In this chapter, the basic concept of RAID and different levels of RAID are explained. Various levels of RAID allow a file and database designer to choose the combination of access efficiency, space utilization, and fault tolerance best suited for the database applications.
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