Ch11: File System Interface
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The document discusses the file system interface, including file concepts, access methods, directory structures, and file operations such as creation, reading, and deletion. It emphasizes the importance of file attributes like name, type, location, and protection, alongside directory organization for efficient file management. Various directory structures, such as single-level, two-level, tree-structured, and acyclic graph directories, are explored, detailing their advantages and challenges in file sharing and protection mechanisms.
Ch11: File System Interface
- 1. Silberschatz, Galvin and Gagne ©200211.1 Chapter 11: File-System Interface File Concept Access Methods Directory Structure File System Mounting File Sharing Protection
- 2. Silberschatz, Galvin and Gagne ©200211.2 File Concept Contiguous logical address space Types: Data numeric character binary Program
- 3. Silberschatz, Galvin and Gagne ©200211.3 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
- 4. Silberschatz, Galvin and Gagne ©200211.4 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.
- 5. Silberschatz, Galvin and Gagne ©200211.5 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.
- 6. Silberschatz, Galvin and Gagne ©200211.6 File Types – Name, Extension
- 7. Silberschatz, Galvin and Gagne ©200211.7 Access Methods Sequential Access read next write next reset no read after last write (rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number
- 8. Silberschatz, Galvin and Gagne ©200211.8 Sequential-access File
- 9. Silberschatz, Galvin and Gagne ©200211.9 Simulation of Sequential Access on a Direct-access File
- 10. Silberschatz, Galvin and Gagne ©200211.10 Example of Index and Relative Files
- 11. Silberschatz, Galvin and Gagne ©200211.11 Directory Structure A collection of nodes containing information about all files. F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes.
- 12. Silberschatz, Galvin and Gagne ©200211.12 A Typical File-system Organization
- 13. Silberschatz, Galvin and Gagne ©200211.13 Information in a Device Directory Name Type Address Current length Maximum length Date last accessed (for archival) Date last updated (for dump) Owner ID (who pays) Protection information (discuss later)
- 14. Silberschatz, Galvin and Gagne ©200211.14 Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system
- 15. Silberschatz, Galvin and Gagne ©200211.15 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 Java programs, all games, …)
- 16. Silberschatz, Galvin and Gagne ©200211.16 Single-Level Directory A single directory for all users. Naming problem Grouping problem
- 17. Silberschatz, Galvin and Gagne ©200211.17 Two-Level Directory Separate directory for each user. •Path name •Can have the same file name for different user •Efficient searching •No grouping capability
- 18. Silberschatz, Galvin and Gagne ©200211.18 Tree-Structured Directories
- 19. Silberschatz, Galvin and Gagne ©200211.19 Tree-Structured Directories (Cont.) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list
- 20. Silberschatz, Galvin and Gagne ©200211.20 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 /mail mkdir count mail prog copy prt exp count Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”.
- 21. Silberschatz, Galvin and Gagne ©200211.21 Acyclic-Graph Directories Have shared subdirectories and files.
- 22. Silberschatz, Galvin and Gagne ©200211.22 Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list ⇒ dangling pointer. Solutions: Backpointers, so we can delete all pointers. Variable size records a problem. Backpointers using a daisy chain organization. Entry-hold-count solution.
- 23. Silberschatz, Galvin and Gagne ©200211.23 General Graph Directory
- 24. Silberschatz, Galvin and Gagne ©200211.24 General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories. Garbage collection. Every time a new link is added use a cycle detection algorithm to determine whether it is OK.
- 25. Silberschatz, Galvin and Gagne ©200211.25 File System Mounting A file system must be mounted before it can be accessed. A unmounted file system (I.e. Fig. 11-11(b)) is mounted at a mount point.
- 26. Silberschatz, Galvin and Gagne ©200211.26 (a) Existing. (b) Unmounted Partition
- 27. Silberschatz, Galvin and Gagne ©200211.27 Mount Point
- 28. Silberschatz, Galvin and Gagne ©200211.28 File Sharing Sharing of files on multi-user systems is desirable. Sharing may be done through a protection scheme. On distributed systems, files may be shared across a network. Network File System (NFS) is a common distributed file- sharing method.
- 29. Silberschatz, Galvin and Gagne ©200211.29 Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List
- 30. Silberschatz, Galvin and Gagne ©200211.30 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7 ⇒ 1 1 1 RWX b) group 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 chgrp G game