A file system is a clearly-defined method that the computer's operating system uses to store, catalog, and retrieve files.

Transcription

1 File Systems

2 A file system is a clearly-defined method that the computer's operating system uses to store, catalog, and retrieve files.

3 Module 11: File-System Interface File Concept Access :Methods Directory Structure Protection Consistency Semantics 11.1

4 Files & File Systems File data, in some format File System Set of named files, maybe organized (directories) Information on files (metadata)

5 Files & File Systems

6 File Concept Contiguous logical address space Types: Data numeric character binary Program 11.2

7 File Types

8 File Types name, extension File Type Usual extension Function Executable exe, com, bin or none ready-to-run machinelanguage program Obje ct obj, o complie d, ma chine language, not linked Source code c, p, pas, 177, asm, a source code in various languages Batch bat, sh commands to the command interpreter Text txt, doc textual data documents Word processor wp, tex, rrf, etc. various word-processor forma ts Library lib, a libraries of routines Print or view ps, dvi, gif, pdf ASCII or binary file Archive arc, zip, tar, gz related files grouped into one file, sometimes compressed. 11.6

9 File Types (a) An executable file (b) An archive 5

10 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 11.3

11 File Metadata

12 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. Applied Operating System Concepts Silberschatz, Galvin, and Gagne!

13 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. Applied Operating System Concepts Silberschatz, Galvin, and Gagne!

14 File Operations create write read reposition within file file seek 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. 11.5

15 Access Methods Sequential Access Direct Access n = relative block number read next write next reset no read after last write (rewrite) read n write n position to n read next write next rewrite n 11.7

16 Sequential-access File 11.44

17 Directories

18 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. 11.8

19 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) 11.9

20 Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system 11.10

21 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 Pascal programs, all games, ) 11.11

22 Single-Level Directory A single directory for all users. Naming problem Grouping problem 11.12

23 Two-Level Directory Separate directory for each user. Path name Can have the same saem file name for different user Efficient searching No grouping capability 11.13

24 Tree-Structured Directories 11.14

25 Tree-Structured Directories (Cont.) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list 11.15

26 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 /spell/mail mkdir count mail prog copy prt exp count Deleting mail deleting the entire subtree rooted by mail

27 Acyclic-Graph Directories Have shared subdirectories and files

28 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

29 Using hard links: Time Machine

30 Using hard links: Time Machine

31 General Graph Directory 11.19

32 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

33 Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List 11.21

34 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access RWX b) groups access RWX c) public access 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 Attach a group to a file chmod 761 game chgrp G game 11.22

35 File-System Structure Allocation Methods Free-Space Management Directory Implementation Efficiency and Performance Recovery 11.23

36 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 a file

37 Typical File-System Organization 11.46

38 File System Implementation A possible file system layout 17

39 Allocation

40 Putting Bytes on Disk File viewed as a contiguous sequence of bytes Allocation is actually storing the bytes

41 Fragmentation Types Data: file not contiguous External: unusable empty space between files Internal: allocated but unused space! file smaller than block

42 Random Access Access time independent of the current block Also called Direct Access RAM: Random Access Memory Tape: no direct access

43 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 (dynamic storage-allocation problem). Files cannot grow. Mapping from logical to physical. LA/512 Block to be accessed =! + starting address Displacement into block = R Q R 11.25

44 Contiguous Allocation of Disk Space

45 Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. block = pointer 11.26

46 Allocate as needed, link together; e.g., file starts at block

47 Linked Allocation (Cont.) Simple need only starting address Free-space management system no waste of space No random access Clusters Mapping of blocks for better performance (disk head Qmoving) to have fewer pointers LA/511 R File-allocation table (FAT): disk-space allocation used by MS-DOS and Block OS/2. to be accessed is the Qth block in the linked chain of The blocks table is representing a list of entries the file. that maps each cluster number to: - the Displacement cluster number into of block the next = R + entry, 1 or - File-allocation an indication table this is (FAT) the last disk-space cluster (end allocation of file), used or by MS- - DOS a special and OS/2. entry to mark bad clusters, or - a 0 to mark the cluster is unused (some cluster may be reserved and are marked in the FAT) 11.28

48 Indexed Allocation Brings all pointers together into the index block. Logical view. index table 11.29

49 Example of Indexed Allocation 11.30

50 Indexed Allocation (Cont.) Need index table Random access Random Dynamic access without external fragmentation, but have overhead of index block. Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 words. We need only 1 block for index table. LA/512 Q = displacement into index table R = displacement into block Q R 11.31

51 Indexed Allocation Mapping (Cont.) M outer-index index table file 11.34

52 Combined Scheme: UNIX (4K bytes per block) 11.35

53 Comparing Allocation Random Access No Data Frag No External Frag Space Waste Contiguous!! " 0 Linked " "! # clusters Indexed! "! > # clusters

54 Free-Space Management Bit vector (n blocks) n-1 bit[i] = block[i] free 10 block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit 11.36

55 Free-Space Management (Cont.) Bit map requires extra space. Example: block size = 2 12 bytes (4K bytes) disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping Counting 11.37

56 Linked Free-Space List on Disk

57 A Quick History of File Systems

58 CP/M File System 1973, Gary Kildall Flat hierarchy (no directory) File name: 8 characters + 3 characters

59 QDOS 1977, Tim Patterson FAT (File Allocation Table) Names: 8 + 3

60 FAT-12 No directories Could address 32 MB (12 bits, 8KB cluster) IBM 5.25 floppy disk: 8 sectors x 40 tracks x 512 bytes = 160 KB Still used on floppies!

61 Evolution of FAT MS-DOS 2.0: directories C:\MYFILES\NOTES DOS 3.31 (1987): FAT bits, 32 KB cluster! 2GB 1995: FAT bits! 8 TB VFAT to display long names

62 On the Mac Side : MFS, 20 MB, 4096 files, no folder hierarchy, 63 characters file names 1985: HFS, 32 GB (later 2TB), hierarchical folders, 31 characters file names MFS and HFS had forks to store metadata icon, associated application 1998: HFS+, 255 characters file names

63 Clusters and Waste HFS: clusters on 16 bits = clusters 1 cluster on a 1 GB drive = 16 KB 1 KB file would take 16 KB HFS+: clusters on 32 bits = 4 billions clusters

64 Unix UFS: case sensitive file names README.TXT! readme.txt block suballocation (merge partially filled blocks together) ext2 (clone of UFS) ext3: ext2 + journaling

65 Journaling To delete a file: Remove its entry in the directory Mark the space as free What if one happens and not the other? 1. Check everything at boot time (fsck) 2. Use a journal to log planned actions

66 Modern File Systems

67 ReiserFS (2001) Journaling Optimized for small files B+-trees Development of Reiser4 slowed because... Hans Reiser has been arrested

68 ReiserFS (2001) Journaling Optimized for small files B+-trees Development of Reiser4 slowed because... Hans Reiser has been arrested

69 NTFS (1993) 64 bits, maximum file size: 2 64 File names in Unicode File dates 01/01/1601 to 28/05/60056 Resolution of 100ns! B+-trees, journaling, extensible metadata Access Control Lists

70 HFS+ (1998) Journaling, extensible metadata Fine-grained permissions (ACL) Tight integration with Mac OS X Spotlight FS-Events

71 ZFS (2004) Developed by Sun Microsystems 128 bits Storage pools instead of partitions Snapshots