1. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 10: File-System Interface

2. 10.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File System and File Abstraction OS provides an abstraction that separates Logical view as seen by the users Actual storage scheme on disk Like main memory, file abstraction gives appearance of contiguous sequence of bytes to user The logical view Abstraction hides potentially non-contiguous representation on physical storage device The physical view

3. 10.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File types File systems often allow us to distinguish between Text files (e.g.: ASCII, Unicode) Binary files (e.g.: Images) Executable files (binaries that we can execute) Storage of each is identical on disk. Bits are bits Interpretation of bits is a program-specific activity File system supports meta-data used to capture the “meaning” of a file so a program can interpret the raw bytes appropriately.

4. 10.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Structure None - sequence of words, bytes Simple record structure Lines Fixed length Variable length Complex Structures Formatted document Relocatable load file Who decides (gives meaning to the sequence of binary data)? Operating system Application program

5. 10.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Attributes Name – information kept in human-readable form Identifier – unique tag (number) identifies file within file system Type – e.g. executable file, text file, media file Location – pointer to file location on storage 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

6. 10.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Types – Name, Extension

7. 10.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Operations File is an abstract data type Create Write Read Reposition within file Delete Truncate Open(F i ) – search the directory structure on disk for entry F i, and move the content of entry to memory Close (F i ) – move the content of entry F i in memory to directory structure on disk

8. 10.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition OS Data Structures to keep track of files Two tables Per-process System wide Per process is used by the process to know what files are open and what their handles are. System-wide table is used to keep track of all open files Locking and protection purposes Prevent operations that would be bad to do if a process is using a file  E.g.: Unmounting filesystem, deleting file, etc…

9. 10.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Open Files Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it Disk location of the file: cache of data access information Access rights: per-process access mode information

10. 10.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

11. 10.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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. 10.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

13. 10.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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, …)

14. 10.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Single-Level Directory A single directory for all users Naming problem Grouping problem

15. 10.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability

16. 10.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories

17. 10.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories (Cont) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list

18. 10.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm Creating a new subdirectory is done in current directory mkdir Example: if in current directory /mail mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

19. 10.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Acyclic-Graph Directories Have shared subdirectories and files

20. 10.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 New directory entry type Link – another name (pointer) to an existing file Resolve the link – follow pointer to locate the file System doesn’t always fix these automatically. E.g.: Deleting target of a symbolic link.

21. 10.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition General Graph Directory

22. 10.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

23. 10.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

24. 10.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition (a) Existing. (b) Unmounted Partition

25. 10.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Mount Point

26. 10.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Sharing Sharing of files on multi-user systems is desirable On distributed systems, files may be shared across a network, e.g. Network File System (NFS) is stnd Unix distributed file-sharing method e.g. CIFS is stnd Windows file sharing protocol Issues Protection, access rights Efficiency Fault tolerance Consistency, synchronization

27. 10.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List

28. 10.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 access1  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. ownergrouppublic chmod761game Attach a group to a file chgrp G game

29. 10.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Windows XP Access-control List Management

30. 10.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Sample UNIX Directory Listing