CS 537 Notes, Section #23: Files, Disk Management

File: a named collection of bits stored on disk. From the OS' standpoint, the file consists of a bunch of blocks stored on the device. Programmer may actually see a different interface (bytes or records), but this does not matter to the file system (just pack bytes into blocks, unpack them again on reading).

Common addressing patterns:

• Sequential: information is processed in order, one piece after the other. This is by far the most common mode: e.g. editor writes out new file, compiler compiles it, etc.
• Random Access: can address any record in the file directly without passing through its predecessors. E.g. the data set for demand paging, also databases.
• Keyed: search for records with particular values, e.g. hash table, associative database, dictionary. Usually not provided by operating system. TLB is one example of a keyed search.

Modern file systems must address four general problems:

• Disk Management: efficient use of disk space, fast access to files, sharing of space between several users.
• Naming: how do users select files?
• Protection: all users are not equal.
• Reliability: information must last safely for long periods of time.

Disk Management: how should the disk sectors be used to represent the blocks of a file? The structure used to describe which sectors represent a file is called the file descriptor.

Contiguous allocation: allocate files like segmented memory (give each disk sector a number from 0 up). Keep a free list of unused areas of the disk. When creating a file, make the user specify its length, allocate all the space at once. Descriptor contains location and size.

• Advantages: easy access, both sequential and random. Simple. Few seeks.
• Drawbacks: horrible fragmentation will preclude large files, hard to predict needs. With interleaved user requests, still cannot eliminate all seeks.

Linked files: In file descriptor, just keep pointer to first block. In each block of file keep pointer to next block. Can also keep a linked list of free blocks for the free list.

• Advantages: files can be extended, no fragmentation problems. Sequential access is easy: just chase links.
• Drawbacks: random access is virtually impossible. Lots of seeking, even in sequential access.
• Example: FAT (MSDOS) file system.

Array of block pointers: file maximum length must be declared when it is created. Allocate an array to hold pointers to all the blocks, but do not allocate the blocks. Then fill in the pointers dynamically using a free list.

• Advantages: not as much space wasted by overpredicting, both sequential and random access are easy.
• Drawbacks: still have to set maximum file size, and there will be lots of seeks.

DOS FAT allocation table: A single File Allocation Table (FAT) that combines free list info and file allocation info. In file descriptor, keep pointer to first block. A FAT table entry contains either (1) the block number of the next block in the file, (2) a distinguished "end of file" (eof) value, or (3) a distinguished "free" value.

• Advantages/Disadvantages: similar to those mentioned above for linked file.

  None of these is a very good solution: what is the answer? First, and MOST IMPORTANT: understand the application. How are file systems used?

  • Most files are small.
  • Much of the disk is allocated to large files.
  • Many of the I/O's are made to large files.

  Thus, the per-file cost must be low, but the performance of large files must be good. Must allow reasonably fast random access, extensibility.

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  Copyright © 1997, 2002 Barton P. Miller
  Non-University of Wisconsin students and teachers are welcome to print these notes their personal use. Further reproduction requires permission of the author.