Direct (Random) Access

• Another method is direct access (or relative access).
• A file is made up of fixed-length logical records that allow programs to read and write records rapidly in no particular order. Thus, we may read block 14, then read block 53, and then write block 7. There are no restrictions on the order of reading or writing for a direct-access file.
• The direct-access method is based on a disk model of a file, since disks allow random access to any file block.
• Direct-access files are of great use for immediate access to large amounts of information. Databases are often of this type.
• For the direct-access method, the file operations must be modified to include the block number as a parameter.
• The block number provided by the user to the OS is normally a relative block number.
  • A relative block number is an index relative to the beginning of the file.
  • Thus, the first relative block of the file is 0, the next is 1, and so on, even though the actual absolute disk address of the block may be 14703 for the first block and 3192 for the second.
• The use of relative block numbers allows the OS to decide where the file should be placed (called the allocation problem) and helps to prevent the user from accessing portions of the file system that may not be part of her file.
• We can easily simulate sequential access on a direct-access file by simply keeping a variable $cp$ that defines our current position, as shown in Fig. 8. Simulating a direct-access file on a sequential-access file, however, is extremely inefficient.

  Figure 8: Simulation of sequential access on a direct-access file.

  
• Modern OSs have all their files are automatically random access.