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Understanding File System Attributes and Operations in Operating Systems

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This chapter provides an in-depth overview of file system attributes, operations, and access methods. Key attributes include file name, type, location, size, protection, and timestamps. It covers essential file operations such as creation, reading, writing, and deletion. The directory structure, either tree-structured or acyclic, is discussed, emphasizing its role in managing file information. Access methods, including sequential and direct access, are explained alongside allocation strategies such as contiguous, linked, and indexed allocation. The chapter also examines permissions and user access levels for efficient file system management.

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Understanding File System Attributes and Operations in Operating Systems

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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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