Chapter 10: File-System

1. Chapter 10: File-System • 10.1 File Concept • 10.2 Access Methods • 10.3 Directory Structure • 10.4 File-System Mounting • 10.5 File Sharing • 10.6 Protection

2. Objectives • To explain the function of file systems • To describe the interfaces to file systems • To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures • To discuss the semantics of sharing files among multiple processes, users, and computers • To explore file-system protection

3. 10.1 File Concept • The operating system abstracts from the physical properties of its storage to define a logical storage unit, the file. • Files are mapped by the OS onto physical, usually nonvolatile, devices. • Use Ultra-Editor to examine contents of a file • File types: • Data, free form or formatted • numeric • character • binary • Program • source • object

4. File Structure • None - sequence of words, bytes • Simple record structure • Lines and Pages • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file • Executable • Who decides: • Operating system • Program

5. File Attributes • Name – only information kept in human-readable form • Identifier – unique tag (number) identifies file within file system • Type – needed for systems that support different file types • Location – pointer to the 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 secondary storage, like a disk

6. File Operations • File is an abstract data type withthe following basic operations • create • write • The system must keep a writer pointer. File info in the directory also updated • read • The system must keep a read pointer. • reposition within file (known as file seek) • delete • truncate • Other operations • append, rename, copy, get/set file attributes

7. Open and Close Files • Most file operations involve searching the directory for a file • 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 • The OS normally maintains two-level open-file tables, per-process and system-wide

8. Open and Close 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

9. Open File Locking • Provided by some operating systems and file systems • Mediates access to a file, like process synchronization • shared lock and exclusive lock • Mandatory or advisory: • Mandatory – access is denied depending on locks held and requested (Windows) • Advisory – processes can find status of locks and decide what to do (Unix)

10. File Locking Example – Java API import java.io.*; import java.nio.channels.*; public class LockingExample { public static final boolean EXCLUSIVE = false; public static final boolean SHARED = true; public static void main(String arsg[]) throws IOException { FileLock sharedLock = null; FileLock exclusiveLock = null; try { RandomAccessFile raf = new RandomAccessFile("file.txt", "rw"); // get the channel for the file FileChannel ch = raf.getChannel(); // this locks the first half of the file - exclusive exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE); /** Now modify the data . . . */ // release the lock exclusiveLock.release();

11. File Locking Example – Java API (cont) // this locks the second half of the file - shared sharedLock = ch.lock(raf.length()/2+1, raf.length(), SHARED); /** Now read the data . . . */ // release the lock sharedLock.release(); }catch (java.io.IOException ioe) { System.err.println(ioe); }finally { if (exclusiveLock != null) exclusiveLock.release(); if (sharedLock != null) sharedLock.release(); } } }

12. Common File Types Unix: use magic number to indicate roughly file types Skip: p.365 第 2 段及第 3 段Skip: 10.1.4, 10.1.5

13. 10.2 Access Methods • Sequential Access (based on tape model) read next write next reset to the beginning no read after last write • Direct Access (or relative access) read n write n position to n read next write next rewrite n (n = relative block number)

14. Simulation of sequential access on a direct-access file Some systems support only one of sequential access and direct access for files. Simulation of direct access on a sequential-access fileis inefficient and clumsy

15. Example of Index and Relative Files Other Access Methods

16. Directory Files F 1 F 2 F 3 F 4 F n 10.3 Directory Structure • A disk may have several partitions. A partition may be with a file system. Several partitions, maybe from many disks, could form a volume that holds a file system. • A collection of nodes containing information about all files • Both the directory structure and the files reside on disk • Backups of these two structures are kept on tapes

17. A Typical File-system Organization

18. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system • for backup (to tape)

19. 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, …

20. Single-Level Directory • A single directory for all users Naming problem Grouping problem

21. Two-Level Directory • Separate directory for each user • Can have the same file name for different user • Isolation or Allow access to other’s files? • If allowed, then use path name • Efficient searching • Use environment variable: search path • No grouping capability

22. Tree-Structured Directories

23. Tree-Structured Directories • Efficient searching • Grouping Capability • Current directory (working directory) • cd /spell/mail/prog • type list • Absolute or relative path name • Creating a new file is done in current directory • Delete a file rm <file-name> • Delete a directory • MS-DOS will not delete a directory unless it is empty • Unix provides an option to delete all files and sub-directories under a directory

24. Tree-Structured Directories • Creating a new subdirectory is done in current directory mkdir <dir-name> Example: if current directory is /mail mkdir count mail prog copy prt exp count In Unix “rm –f mail”  deleting the entire subtree rooted by “mail”

25. Acyclic-Graph Directories • Use link to have shared subdirectories and files • Another approach: duplicate all information about subdirectories and files in both sharing directories. But it is hard to maintain consistency when a shared file is modified.

26. Acyclic-Graph Directories • New directory entry type • Link – another name (pointer) to an existing file • Resolve the link – follow pointer to locate the file • Two different names (aliasing) • A file could have multiple absolute path names. • Traverse problem. • If dict deletes all dangling pointer. Solutions: • Just wait for users to find out. It is used with symbolic links: • Preserve the file until all references to it are deleted. Unix uses this approach for hard links by keeping a reference count in the file information block. • Acyclic-graph could be maintained by prohibiting multiple references to directories SKIP: 10.3.7