Chapter 10: File-System Interface

1. Chapter 10: File-System Interface

2. Chapter 10: File-System Interface • File Concept • Access Methods • Directory Structure • File-System Mounting • File Sharing • Protection

3. 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 explore file-system protection

4. File Concept • The OS must provide a uniform logical view of information storage. • OS abstracts from the physical properties of its storage devices • Defines a logical storage unit, the file • Files are mapped by the OS onto physical devices. • A file is a named collection of related information • Recorded on secondary storage • Files represent both programs and storage • Contiguous logical address space • Types: • Data • numeric • character • binary • Program (source) • Object file (organized to be recognizable by linker) • Executable file

5. 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

6. 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 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

7. File Operations • File is an abstract data type • Create. 1. Find space in the file system. 2. Create entry in the directory. • Write. Need to maintain a write pointer. • Read. Need to maintain a current-file-position pointer. • Reposition within file • 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

8. 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

9. Open File Locking • Provided by some operating systems and file systems • Mediates access to a file • Mandatory or advisory: • Mandatory – access is denied depending on locks held and requested • Advisory – processes can find status of locks and decide what to do

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 exclusiveLock.release(); }catch (java.io.IOException ioe) { System.err.println(ioe); }finally { if (exclusiveLock != null) exclusiveLock.release(); if (sharedLock != null) sharedLock.release(); } } }

12. File Types – Name, Extension

13. 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

14. Sequential-access File

15. Simulation of Sequential Access on a Direct-access File

16. Access Methods • Other Access Methods • Can build other access methods on top of a direct-access method • Generally construct an index for the file. • Index contains pointers to blocks • To find a record, search the index, then use the pointer

17. Access Methods • Example: Retail-price file • Lists universal product codes (UPCs) and associated prices • Each record: 10-digit UPC, 6-digit price (16 byte total) • A file of 120,000 records would take 2,000 blocks (2 million bytes). • Keep file sorted by UPC, then can define an index consisting of the first UPC in each block. • The index would have 2,000 entries of 10 digits or 20,000 bytes • Could keep in memory. • To find the price of an item, binary search the index.

18. Example of Index and Relative Files

19. Access Methods • Problem: index file may become too large to keep in memory • Solution: index the index file. • Example: IBM’s indexed sequential-access method (ISAM) • Small master index • That points to disk blocks of a secondary index • Secondary index points to actual file blocks • Keep sorted on a defined key • To find an item: • Binary search master index, get block number of secondary index • Read in block, perform binary search on secondary index • Determine desired block from this search and read it in • Search this block sequentially to find record.

20. 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

21. Directory Structure • Disk can be used in its entirety for a file system (FS) • Or can partition a disk to contain different FS • May also combine partitions into volumes and create FS on these • Each volume that contains a FS must contain info about the files in the system • Kept in entries in a device directory or volume table of contents. Usually just called a directory. • See next slide

22. A Typical File-system Organization

23. Directories • A directory is like a symbol table that translates file names into their directory entries. • Directories can be organized in many ways. • Must allow operations on directories

24. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

25. 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, …)

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

27. Two-Level Directory • Separate directory for each user • Path name • Can have the same file name for different user • Efficient searching • No grouping capability

28. Tree-Structured Directories

29. Tree-Structured Directories (Cont) • Efficient searching • Grouping Capability • Sharing of files is not allowed • New information that file manager will need: • Current directory (working directory) • Directory that user process is currently working in • Path information: what directory paths to search to find commands and programs

30. mail prog copy prt exp count 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 Deleting “mail”  deleting the entire subtree rooted by “mail” rmdir mail or rm –r mail

31. Acyclic-Graph Directories • Have shared subdirectories and files No cycles allowed!

32. Acyclic-Graph Directories (Cont.) • Two different names (aliasing) may point to same file • Don’t want to traverse shared structures more than once • One solution: Ignore links when traversing directories to avoid cycles • If dict deletes list dangling pointer • If the file space is reused, the dangling pointer may point to this file • Unix lets the user deal with this problem Solutions: • Backpointers, so we can delete all pointersVariable size records a problem: how many pointers do you store? • 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

33. General Graph Directory • Have shared subdirectories and files cycles allowed!

34. General Graph Directory (Cont.) • How do we deal with cycles? • Allow only links to file not subdirectories (would prevent cycles) • Garbage collection • Traverse entire file system, mark everything that can be accessed • Second pass collects everything that is not marked into a list of free space • Very time consuming • Every time a new link is added use a cycle detectionalgorithm to determine whether it is OK • Bypass links when during directory traversal

35. File System Mounting • A file system must be mounted before it can be accessed • Directory structure for the new volume must be built and added to the FS • Mounting • First, choose the mount point: the location within the file structure where new file system is to be attached • Typically an empty directory • Example: /home in Linux • To access directory structure within that file system, precede the directory names with the mount point • Example: /home/barr, /home/poore, /home/kropa

36. File System Mounting • A file system must be mounted before it can be accessed • Directory structure for the new volume must be built and added to the FS • Mounting • Next, file manager verifies that the device contains a valid FS • Asks device driver to read the device directlry • Verifies that the directory has the expected format • Finally, OS notes in its directory structure that a file system is mounted at the specified mount point

37. File System Mounting • Example (see next slide) • Triangles represent subtress • Figure a is the existing FS, figure b is unmounted vlume on /device/dsk. • Only files on existing FS can be accessed.

38. (a) Existing. (b) Unmounted Partition

39. File System Mounting • Example (see next slide) • New FS is mounted over /users. • Existing users are lost or obscured

40. Mount Point

41. Mount Points • Mac OS • When OS encournters a disk for the first time, searches for a FS on the device. • If it finds one, will automatically mount the FS at the root level • Adds a folder icon on the screen labeled with the name of the FS (as stored in the device directory)

42. Mount Points • Windows family (95 through XP) • Maintains an extended two-level directory structure • Devices and volumes are asigned drive letters • Volumes have a general graph directory structure assoicated with the drive letter: • Drive-letter:\path\to\file • More recent versions allow a FS to be mounted anywhere in the directory tree (similar to UNIX). • Windows OS automatically discover all devices and mount at boot time

43. Mount Points • UNIX • Mount commands are explicit. • A ssytems config file contains a list of devices and mount points for automatic mounting at boot time • Other mounts are executed manually.

44. 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

45. File Sharing – Multiple Users • Issues: • File sharing • File naming • File protection

46. File Sharing – Multiple Users • System must mediate the file sharing • Must maintain more file and directory attributes than a single-user system • Most systems use two concepts: • file (or directory) owner (or user) • Group • The owner can change attributes and grant access • The group attribute defines a subset of users who can share access to the file (or directory)

47. File Sharing – Multiple Users • When a user requests an operation on a file, file manager • Compares the user ID of the use requesting the operation with the owner attribute of the file • Then compares group IDs • Then can apply appropriate permissions

48. File Sharing – Multiple Users • New attributes necessay: • User IDs identify users, allowing permissions and protections to be per-user • Group IDs allow users to be in groups, permitting group access rights

49. File Sharing – Remote File Systems • Uses networking to allow file system access between systems • Manually via programs like FTP • Anonymous (no account on remote machine). • www uses this almost exclusively • Authenticated. • Used in DFS • Automatically, seamlessly using distributed file systems (DFS) • Semi automatically via theworld wide web • Need a browser • Use separate operations (essentially a wrapper for ftp) to transfer files

50. File Sharing – Remote File Systems • Client-server model allows clients to mount remote file systems from servers • Machine containing the files is the server • Machine seeking access is the client • Server can serve multiple clients/client can use multiple servers • Client identification and authentication difficult • Network names and IP addresses can be spoofed • Can use encrypted keys; need to ensure same algm is used in client and server