1. MODERN OPERATING SYSTEMS Third Edition ANDREW S
MODERN OPERATING SYSTEMS Third Edition ANDREW S. TANENBAUM Chapter 4 File Systems
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

2. File Systems (1)
Essential requirements for long-term information storage:
It must be possible to store a very large amount of information.
The information must survive the termination of the process using it.
Multiple processes must be able to access the information concurrently.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

3. File Systems (2)
Think of a disk as a linear sequence of fixed-size blocks and supporting reading and writing of blocks. Questions that quickly arise:
How do you find information?
How do you keep one user from reading another’s data?
How do you know which blocks are free?
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

4. files
Processes (threads), address spaces, files are the most important concepts in OS
Files are logical units of information created by processes
Similar to kind of address space
File system
Manage files: how they are structured, named, accessed, used, protected, implemented, etc

5. File naming Files are abstraction mechanism Two-part file name
To store information on the disk and read it back
When a process creates a file, it gives the file a name; and the file can be accessed by the name
Two-part file name
File extension: indicating characteristics of file
In Unix, file extension is just convention; C compiler is exception
In windows, file extensions specify which program “owns” that extension; when double clicking, program assigned to it is launched

6. Figure 4-1. Some typical file extensions.
File Naming
Figure 4-1. Some typical file extensions.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

7. File Structure
Figure 4-2. Three kinds of files. (a) Byte sequence. (b) Record sequence. (c) Tree.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

8. File Types Regular Files: Directory
ASCII files or binary files
ASCII consists of lines of text; can be displayed and printed
Binary, have some internal structure known to programs that use them
Directory
Files to keep track of files
Character special files (a character device file)
Related to I/O and model serial I/O devices
Block special files (a block device file)
Mainly to model disks

9. File Types Figure 4-3. (a) An executable file. (b) An archive.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

10. File Access File descriptor
A file descriptor is a small integer representing a kernel-managed object that a process may read from or write to
Every process has a private space of file descriptors starting at 0
By convention, 0 is standard input, 1 is standard output, and 2 is standard error
System call: read() and write() to read from and write to filles named by file descriptors

11. Figure 4-4a. Some possible file attributes.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

12. Struct stat Struct stat{
mode_t st_mode; // file type and mode (permission)
ino_t st_ino; //inode number
dev_t st_dev; //device number
dev_t st_rdev; //device number (special)
nlink_t st_nlink; //number of links
uid_t st_uid; // user ID of owner
gid_t st_gid; // group ID of owner
off_t st_size; //size in bytes
time_t st_atime; //time of last access
……}

13. File attributes drwxr-xr-x 2 root root 4096 Sep 24 2008 Unit2
drwxr-xr-x 2 root root May 26 19:21 a
-rwxr-xr-x 1 root root Aug 5 22:49 a.out
-rwxrwx--T 1 root root 81 Aug a.txt
-rwxr-x--- 1 root root 81 May 26 19:20 b.txt
-rwx root root 81 Jul 30 19:28 c.txt
-rwxr-xr-x 1 root root Jul 30 19:27 cp

14. File Owner
Group Owner
Everyone Else
Write Permission
Read Permission
Execute
Permission

15. UNIX
File Owner
Group Owner
Everyone Else

16. File Operations The most common system calls relating to files: Create
Delete
Open
Close
Read
Write
Append
Seek
Get Attributes
Set Attributes
Rename
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

17. Example Program Using File System Calls (1)
. . .
Figure 4-5. A simple program to copy a file.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

18. Example Program Using File System Calls (2)
Figure 4-5. A simple program to copy a file.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

19. Hierarchical Directory Systems (1)
Single-level directory system:
The simpliest
Figure 4-6. A single-level directory system containing four files.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

20. Hierarchical Directory Systems (2)
Figure 4-7. A hierarchical directory system.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

21. Figure 4-8. A UNIX directory tree.
Path Names
Figure 4-8. A UNIX directory tree.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

22. Path names Absolute path name Relative path name
Start from root and are unique
Relative path name
Current working set: all path names not beginning at the root directory are taken relative to the working directory
If current working directory is /usr/ast:
then cp /usr/ast/mailbox /usr/ast/mailbox.bak
Is: cp mailbox mailbox.bak

23. Path names Special Entries: “.” and “..”
Dot refers to the current directory; dotdot refers to the parent directory (except root)
cp /usr/lib/dictionary .
cp /usr/lib/dictionary dictionary
Cp /usr/lib/dictionary /usr/ast/dictionary

24. Directory Operations System calls for managing directories: Create
Delete
Opendir
Closedir
Readdir
Rename
Link
Unlink
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

25. Directory operation Hard link Symbolic link
Linking allows a file to appear in more than one directory; increments the counter in the file’s i-node
Symbolic link
A name is created pointing to a tiny file naming another file

26. File System Implementation
Users:
How files are names, what operations are allowed on them, what the directory tree looks like
Implementors
How files and directories are stored, how disk space is managed and how to make every thing work efficiently and reliably

27. File System Layout File system are stored on disks.
Most disks are divided up into several partitions
Sector 0 is called MBR (master boot record), to boot the computer
BIOS reads in and executes MBR, MBR locates the active partition, reads in the boot block, and execute
The boot block reads in the OS contained in the partition

28. Figure 4-9. A possible file system layout.
Superblock: contains all the key parameters about a file system; read into memory the booted or the FS is used
Figure 4-9. A possible file system layout.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

29. Contiguous Allocation
Figure (a) Contiguous allocation of disk space for 7 files.
(b) The state of the disk after files D and F have been removed.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

30. Contiguous Allocation
Advantage：
Simple to implement; to record the first block and length
Easy to read; only one seek is needed
Disadvantage:
Fragmentation

31. Linked List Allocation
Figure Storing a file as a linked list of disk blocks.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

32. Linked list allocation
To keep each file as a linked list of disk blocks
Advantage
Only internal fragmentation
Disadvantage
Random access is difficult
Adding a pointer at the head of block; extra overhead while copying

33. Linked List Allocation Using a Table in Memory
Figure Linked list allocation using a file allocation table in main memory.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

34. Linked List Allocation Using a Table in Memory
FAT-File Allocation Table
Advantage
Can take use of the whole block
Random access is easy
only to store the starting block number
Disadvantage
To keep the entire table in memory
Can’t scale well

35. I-nodes Figure 4-13. An example i-node.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

36. i-nodes Advantage Disadvantage
i-node need only be in memory when the corresponding file is open; file table grows linearly with the disk
Disadvantage
Each i-node has fixed size

37. Implementing Directories (1)
Figure (a) A simple directory containing fixed-size entries with the disk addresses and attributes in the directory entry. (b) A directory in which each entry just refers to an i-node.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

38. Implementing Directories (2)
Figure Two ways of handling long file names in a directory. (a) In-line. (b) In a heap.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

39. Shared Files (2)
Figure (a) Situation prior to linking. (b) After the link is created. (c) After the original owner removes the file.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

40. Figure 4-20. Percentage of files smaller than a given size
Disk Space Management Block Size (1)
Figure Percentage of files smaller than a given size
(in bytes).
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

41. Disk Space Management Block Size (2)
Figure The solid curve (left-hand scale) gives the data rate of a disk. The dashed curve (right-hand scale) gives the disk space efficiency. All files are 4 KB.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

42. Keeping Track of Free Blocks (1)
Figure (a) Storing the free list on a linked list. (b) A bitmap.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

43. Keeping Track of Free Blocks (2)
Figure (a) An almost-full block of pointers to free disk blocks in memory and three blocks of pointers on disk. (b) Result of freeing a three-block file. (c) An alternative strategy for handling the three free blocks. The shaded entries represent pointers to free disk blocks.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

44. The MS-DOS File System (1)
Figure The MS-DOS directory entry.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

45. FAT versions FAT comes up with 3 versions: FAT 12 FAT 16 FAT 32
Differ in how many bits a disk address contains
Block size also varies: 512B, 1K, 2K,4K,…,32K

46. The MS-DOS File System (2)
Figure Maximum partition size for different block sizes. The empty boxes represent forbidden combinations.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

47. The UNIX V7 File System (1)
Figure A UNIX V7 directory entry.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

48. The UNIX V7 File System (2)
Figure A UNIX i-node.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

49. The UNIX V7 File System (3)
Figure The steps in looking up /usr/ast/mbox.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

50. exercises
Consider a disk with 1 MB per track, a rotation time of 8.33 msec, and an average seek time of 5 msec. Then what is the time to read a block of k bytes?

51. exercise
Consider the idea behind Fig.4-21, for a disk with a mean seek time of 8msec, a rotational rate of 15,000 rpm, and 264,144 bytes per track. What are the data rate for block sizes of 1KB, 2KB and 4KB, respectively?

52. Exercise
How many disk operations are needed to fetch the i-node for the file /usr/ast/courses/os/handout.t? Asume that the i-node for the root directory is in memory, but nothing else along the path is in memory. Also assume that all directories fit in one disk block.