File-Systems

1. File-Systems • File Concept • Access Methods • Directory Structure • Protection • File-System Structure • Allocation Methods • Free-Space Management • Directory Implementation • Efficiency and Performance • Recovery • borrowed from Silbeezrschatz, Galvin and Gagne

2. File Concept • Named collection of related information on secondary storage • Types: • Data • numeric • character • binary • Program

3. File Structure • None - sequence of words, bytes • Simple record structure • Lines, Fixed length, Variable length • Complex Structures • Formatted document, multi-media documents • Who decides: • Operating system • Application • “Middleware” • DBMS

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. 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. File Types – name, extension

7. Access Methods • Sequential Access - • read next • write next • reset • no read after last write • (rewrite) • Direct Access: n = relative block number • read n • write n • position to n • read next • write next • rewrite n

8. 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.

9. 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)

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

11. Organize the Directory (Logically) • 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, …)

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

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

14. Tree-Structured Directories

15. Tree-Structured Directories • Efficient searching • Grouping Capability • Current directory (working directory) • cd /spell/mail/prog • type list

16. Tree-Structured Directories • 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’

17. Acyclic-Graph Directories • Have shared subdirectories and files.

18. Acyclic-Graph Directories • 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.

19. General Graph Directory

20. 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.

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

22. Access Lists and Groups • Mode of access: read, write, execute • Three classes of users RWX a) owner access 7  1 1 1RWX 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 particular file or subdirectory, define an appropriate access. owner group public chmod 761 game Attach a group to a file chgrpG game

23. File-System Structure • File structure • Logical storage unit • Collection of related information • File system resides on secondary storage (disks). • File system organized into layers. • File control block – storage structure consisting of information about an open file.

24. Contiguous Allocation • Each file occupies a set of contiguous blocks on the disk. • Simple – only starting location (block #) and length (number of blocks) are required. • Random access. • Wasteful of space external fragmentation, may use compaction to fix. • Files cannot grow. • Mapping from logical to physical.

25. Linked Allocation • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block = Allocate as needed, link together; e.g., file starts at block 9

26. Linked Allocation (Cont.)

27. Linked Allocation (Cont.) • Simple – need only starting address • Free-space management system – no waste of space • No random access • Mapping

28. Indexed Allocation • Brings all pointers together into the index block. • Logical view. index table

29. Example of Indexed Allocation

30. Indexed Allocation (Cont.) • Need index table • Random access • Dynamic access without external fragmentation, but have overhead of index block. • Wasted space • Index levels

31. Indexed Allocation – Mapping  outer-index file index table

32. Combined Scheme: UNIX

33. Free-Space Management • Bit vector (n blocks) 0 1 2 n-1 … 0  block[i] free 1  block[i] occupied bit[i] = 

34. Free-Space Management (Cont.) • Bit map requires extra space. Example: block size = 212 bytes (4096 Bytes) disk size = 234 bytes (16 GByte)n = 234/212 = 222 bits (4Mbits=512 KBytes) • Linked list (free list) • Cannot get contiguous space easily • No waste of space • Grouping • Counting

35. Free-Space Management (Cont.) • Need to protect: • Pointer to free list • Bit map • Must be kept on disk • Copy in memory and disk may differ. • Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk. • Solution: • Set bit[i] = 1 in disk. • Allocate block[i] • Set bit[i] = 1 in memory

36. Directory Implementation • Linear list of file names with pointer to the data blocks. • simple to program • time-consuming to execute • Hash Table – linear list with hash data structure. • decreases directory search time • collisions – situations where two file names hash to the same location • fixed size

37. Efficiency and Performance • Efficiency dependent on: • disk allocation and directory algorithms • types of data kept in file’s directory entry • Performance • disk cache – separate section of main memory for frequently sued blocks • free-behind and read-ahead – techniques to optimize sequential access • improve PC performance by dedicating section of memory as virtual disk, or RAM disk.

38. Various Disk-Caching Locations

39. Recovery • Consistency checker – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies. • Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape). • Recover lost file or disk by restoring data from backup.

40. File System Implementations • UNIX Examples - SVR4 and BSD

41. UNIX FS Framework Provides persistent storage Facilities for managing data Interface exported abstractions: files, directories, file descriptors and file systems kernel does not interpret file contents files and directories form tree structure

42. File and Directory Organization / (hard) links bin etc dev usr vmunix sh local etc /usr/local/bin/bash bin bash

43. File Attributes Type - for example regular, FIFO, special. Reference count size in bytes device id ownership access modes timestamps

44. User View of Files • File Descriptors (open, dup, dup2, fork) • All I/O is through file descriptors • references the open file object • per process object • File Object - holds context • created by an open() system call • stores file offset • reference to vnode • vnode - abstract representation of a file

45. How it works File Descriptors {{0, uf_ofile} {1, uf_ofile} {2 , uf_ofile} {3 , uf_ofile} {4 , uf_ofile} {5 , uf_ofile}} Open File Objects {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}} Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file

46. File Systems File hierarchy composed of one or more File Systems One File System is designated the Root File System Attached to mount points File can not span multiple File Systems resides on one logical disk

47. Logical Disks Viewed as linear sequence of fixed sized, randomly accessible blocks. A file system must reside in a logical disk, however a logical disk need not contain a file system. Typically physical disks divided into partitions that correspond to logical disks

48. FS Overview System calls vnode interface /proc tmpfs UFS HSFS PCFS RFS NFS swapfs cdrom diskette disk Process address space Anonymous memory Example from Solaris

49. Local Filesystems S5fs - System V file system. Based on the original implementation. FFS/UFS - BSD developed filesystem with optimized disk usage algorithms

50. S5fs - Disk layout Viewed as a linear array of blocks Typical disk block size 512, 1024, 2048 bytes Physical block number is the block’s index disk uses cylinder, track and sector first few blocks are the boot area, which is followed by theinode list (fixed size)