Q7Operating System

A file access method defines the manner in which data in a file can be accessed. The main file access methods are:

• Sequential Access: The simplest access method. Information in the file is processed in order, one record after another. Read advances a file pointer; write appends to the end of file. Most common for text files and streams. Example: Tape drives, log files. Rewinding/backtracking is allowed but not efficient for random access.
• Direct (Random) Access: A file is made of fixed-length logical records that allow programs to read and write records rapidly in no particular order. The file is viewed as a numbered sequence of blocks/records. Operations: read(n) and write(n) where n is the block number. Efficient for applications like databases and airline reservations that need fast random record retrieval.
• Indexed Access: Built on top of direct access. Maintains an index file that contains pointers to the various blocks of the main file. To find a record, search the index for the pointer, then use it to directly access the main file block. Allows fast searching by key. Used in database systems and file systems (e.g., ISAM — Indexed Sequential Access Method).
• Indexed Sequential Access (ISAM): Combines sequential and indexed access. Records are stored sequentially; an index of key fields provides direct access to any record. Supports both sequential and random retrieval efficiently.

The choice of access method depends on the application. Most OS provide sequential and direct access at the file system level; applications implement indexed access. UNIX treats all files as sequences of bytes supporting both sequential and direct (lseek) access.