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Operating Systems Chapter 4

Operating Systems Chapter 4. The File System. File Attributes. Name – only information kept in human-readable form. Type – needed for systems that support different types. Location – pointer to file location on device. Size – current file size.

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Operating Systems Chapter 4

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  1. Operating SystemsChapter 4 The File System

  2. File Attributes • Name – only information kept in human-readable form. • Type – needed for systems that support different types. • Location – pointer to file location on device. • Size – current file size. • Protection – controls who can do reading, writing, executing. • Time, date, and user identification – data for protection, security, and usage monitoring. Information about files are kept in the directory structure, which is maintained on the disk.

  3. File Operations • create • write • read • reposition within file – file seek • delete • truncate • open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory. • close (Fi) – move the content of entry Fi in memory to directory structure on disk.

  4. Access Methods • Sequential Access read next write next reset • Direct Access (Random) read n write n position to n n = relative block number

  5. Directory Files F 1 F 2 F 3 F 4 F n Directory Structure • A collection of nodes containing information about all files.

  6. Tree-Structured Directories

  7. Acyclic-Graph Directories

  8. Acyclic-Graph Directories (Cont.) • Two different names (aliasing) • If dict deletes w/list  dangling pointer. Solutions: • Backpointers, so we can delete all pointers -> Variable size records a problem. • Entry-hold-count solution.

  9. Acyclic-Graph Directories (Cont.) • How do we guarantee no cycles? • Allow only links to file not subdirectories. • Every time a new link is added use a cycle detectionalgorithm to determine whether it is OK.

  10. group owner public chmod 761 game Access Lists and Groups • Mode of access: read, write, execute • Three classes of users RWX a) owner access 7  1 1 1 RWX b) groups access 6  1 1 0 RWX c) public access 1  0 0 1 • Attach a group to a file chgrp G game

  11. In-Memory File System Structures

  12. File-System Allocation Methods • Contiguous Allocation • Linked Allocation • Indexed Allocation

  13. Contiguous Allocation

  14. Linked Allocation • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. • Simple – need only starting address • Free-space management system – no waste of space • No random access • Allocate as needed, link together; e.g., file starts at block 9

  15. Linked Allocation (cont.)

  16. File-Allocation Table • File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2.

  17. index table Indexed Allocation • Brings all pointers together into the index block.

  18. Example of Indexed Allocation

  19. Indexed Allocation (Cont.) • Need index table • Random access • have overhead of index block. • Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 words. We need only 1 block for index table.

  20. UNIX inode (UFS)

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