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File Management System Objectives

File Management System Objectives. Meet the data management needs of the user Guarantee that the data in the file are valid Optimize performance Provide I/O support for a variety of storage device types Minimize the potential for lost or destroyed data

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File Management System Objectives

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  1. File Management System Objectives • Meet the data management needs of the user • Guarantee that the data in the file are valid • Optimize performance • Provide I/O support for a variety of storage device types • Minimize the potential for lost or destroyed data • Provide a standardized set of I/O interface routines to user processes • Provide I/O support for multiple users in the case of multiple-user systems Operating System Concepts

  2. File Structure • None - sequence of words, bytes • Simple record structure • Lines • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file • Can simulate last two with first method by inserting appropriate control characters. • Who decides: • Operating system • Program Operating System Concepts

  3. 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. Operating System Concepts

  4. File Operations • create • open • write • read • seek • delete • Truncate • close Operating System Concepts

  5. File Types – name, extension Operating System Concepts

  6. Access Methods • Provides a standard interface between applications and the file systems and devices that hold the data • Different access methods reflect different file structures and different ways of accessing and processing the data Operating System Concepts

  7. Sequential Access • Information is accessed in the order it is stored in the file. • Read sequentially. • Key field uniquely identifies the record & determines storage order • Appended at the end of file • Typically used in batch applications Operating System Concepts

  8. Indexed Sequential File • Adds an index to the file to support random access • Adds an overflow file • Greatly reduces the time required to access a single record • Multiple levels of indexing can be used to provide greater efficiency in access Operating System Concepts

  9. Direct Access Method • Read and Write Randomly • Typically used in Hard Disk rather than magnetic tapes. Operating System Concepts

  10. Directory Structure • A collection of nodes containing information about all files. Directory Files F 1 F 2 F 3 F 4 F n • Both the directory structure and the files reside on disk. • Backups of these two structures are kept on tapes. Operating System Concepts

  11. Information in a Device Directory • Name • Type • Address • Current length • Maximum length • Date last updated (for dump) • Owner ID • Protection information Operating System Concepts

  12. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system • Link and Unlink Operating System Concepts

  13. Organize the Directory (Logically) to Obtain • Efficiency – locating a file quickly. • Naming – convenient to users. • Two users can have same name for different files. • The same file can have several different names. • Grouping – logical grouping of files by properties, (e.g., all Pascal programs, all games, …) Operating System Concepts

  14. Single-Level Directory • A single directory for all users. • Naming problem • Grouping problem Operating System Concepts

  15. Two-Level Directory • Separate directory for each user. Operating System Concepts

  16. Tree-Structured Directories Operating System Concepts

  17. Tree-Structured Directories (Cont.) • Absolute or relative path name • Creating a new file is done in current directory. • Delete a file rm <file-name> • Creating a new subdirectory is done in current directory. mkdir <dir-name> Example: if in current directory /spell/mail mkdir count mail prog copy prt exp count • Deleting “mail”  deleting the entire subtree rooted by “mail”. Operating System Concepts

  18. Acyclic-Graph Directories • Have shared subdirectories and files. Operating System Concepts

  19. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List Operating System Concepts

  20. 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 • Ask manager to create a group (unique name), say G, and add some users to the group. • For a particular file (say game) or subdirectory, define an appropriate access. owner group public chmod 761 game • Attach a group to a file • chgrpG game Operating System Concepts

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