1. Long-term Information Storage
Must store large amounts of data
Information stored must survive the termination of the process using it
Multiple processes must be able to access the information concurrently. In short:

2. Long-term Information Storage
Files: Good!

3. Long-term Information Storage
Files: Good!
No Files: Bad!

4. File System Operating system determines how files are:
Structured
Named
Accessed
Used
Protected
Implemented
Most important aspect to users is how files appear to them: naming convention, available operations, protection, etc. (Not implementation!!).

5. File Naming Unix: Windows: Case sensitive.
Allows, but does not require, extensions (e.g., prog.c).
Assigns no meaning to extensions.
Add as many extensions as desired (e.g., prog.back.stupid.c).
Does not allow spaces in name (unless “\ “) ;
Windows:
Not case sensitive.
Allows 1-3 character extensions.
Extensions have meaning (to other application codes, not to the OS)
Allows spaces in file name. (ever tried to copy “my work” to Unix?)

6. Typical file extensions.
File Naming
Typical file extensions.

7. File Structure None - sequence of words, bytes Simple record structure
Lines
Fixed length
Variable length
Complex Structures
Formatted document
Relocatable load file

8. File Structure Three kinds of files
byte sequence (i.e., no structure).
record sequence
Tree (e.g., data base)

9. File Structure
Can simulate last two with first method by inserting appropriate control characters
Who decides:
Operating system
Program (i.e., programs can support any model they want)
Unix and Windows support only the sequence of bytes functionality.

10. An executable file (Unix)

11. File Access Sequential access Random access
read all bytes/records from the beginning
cannot jump around, could rewind or back up
convenient when medium was mag tape
Random access
bytes/records read in any order
essential for data base systems
read can be …
move file marker (seek), then read or …
read and then move file marker

12. File Attributes Name – only information kept in human-readable form
Identifier (file descriptor) – 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

13. File Attributes
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 (although generally cached).

14. File Operations Create Write Read Reposition within file Delete
Truncate

15. File Operations in Unix
int fd = open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory
fd is a file descriptor (integer).
close (fd) – move the content of entry Fi in memory to directory structure on disk
seek() // change pointer to current location in file.
read(fd, buf, num_bytes)

16. 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
Open-file 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

17. Open Files
Unix maintains an open-file table for each process and for the whole system.
File descriptor is used as an index into the process open-file table. Entries are items that have to do with that particular process (e.g., file pointer, access rights, etc.).
A pointer to the system-wide open-file table is also in the process open-file table.
System-wide open-file table holds process-independent information (e.g., location on disk, last access time, file size, count of the number of processes using the file).

18. 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

19. Directory Both the directory structure and the files reside on disk
A collection of data structures containing information about 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

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

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 Java programs, all games, …)

22. Single-Level Directory
Naming problem
Grouping problem

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

24. Tree-Structured Directories

25. Tree-Structured Directories (Cont)
Efficient searching
Grouping Capability
In Unix, a directory is a file that contains meta-data about the files it contains.

26. Tree-Structured Directories (Cont)
Most OS support absolute and relative path names.
Unix has two pre-defined relative path names:
. Represents current directory
.. Represents parent directory
Current directory (working directory)
cd /spell/mail/prog or
cd .. (relative to CD)

27. Path Names
A UNIX directory tree

28. To Open dict, the absolute path name is: /usr/lib/dict.

29. Relative Path Name
Assume Current Directory is /usr/jim. Then .. is /usr,
. is /usr/jim
To access dict: ../lib/dict.

30. rmdir removes an entire directory (and all sub-directories).
Unix: mkdir creates a new sub-directory below the current working directory.
rmdir removes an entire directory (and all sub-directories).
rm deletes a file
If someone suggests that you try out a cool command called
rm –r * don’t do it!!

31. File system containing a shared file
Shared Files (1)
File system containing a shared file

32. Links
This is termed a hard link. Both directory entry pointing to the same inode.
(a) Situation prior to linking
(b) After the link is created
(c) After the original owner removes the file

33. Symbolic Links
Provide the path name of the target file in the linked file.
Other processes do not have access to the inode (i.e., directory structure).
What happens when file deleted by owner?

34. Implementing Directories (1)
(a) A simple directory fixed size entries
disk addresses and attributes in directory entry
(b) Directory in which each entry just refers to an i-node

35. File Control Block

36. Accessing a File

37. Allocation of File Blocks
Contiguous allocation
Linked-list allocation
FAT
Indexed (inodes).

38. Directory Structure with Contiguous Allocation of File Blocks

39. Implementing Files: Contiguous Allocation
(a) Contiguous allocation of disk space for 7 files
(b) State of the disk after files D and E have been removed

40. Linked-list Allocation

41. File Allocation Table

42. Entry 4 bytes. Blocks 1K. 20 Million Entries (not files
Entry 4 bytes. Blocks 1K. 20 Million Entries (not files!) == 80 MB for table.

43. Indexed Allocation

44. Unix inode

45. Unix Directory Entry
File Name Inode
Tester

46. Unix File System Unix File System: 1 inode for each file/directory. B
Inode list
Data blocks
Boot area
superblock

47. Pointing to file data blocks Pointing to file data blocks
File Attributes
Direct blocks
Pointing to file data blocks
Pointing to file data blocks
Single Indirect
Double and Triple Indirect blocks not shown
Points to data block whose data is the addresses of data blocks belonging to the file

48. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Problem: Open file /usr/pmd

49. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Open file /usr/pmd
Step 1: Fetch inode for / (root, always inode 0)

50. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Open file /usr/pmd
File Attributes
100
400
Inode 0

51. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Step 2: Grab data block 100 (it had better be a directory!!)

52. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
bin
include 6
usr
data block 100 (directory listing for /)

53. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
bin
include 6
usr
Step 3: Grab inode 2.
data block 100 (directory listing for /)

54. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
File Attributes
180
Direct block 1
Step 4: Grab Data block 180
inode 2

55. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Step 4: Grab Data block 180

56. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
weew
Pops
pmd
data block 180. Directory listing for /usr

57. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Step 5: Grab inode 3
weew
Pops
pmd
Data block 180. Directory listing for /usr

58. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
Inode Inode Inode Inode 3
Root inode
File Attributes
253
Direct block 1
inode 3

59. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct block 1
Inode Inode Inode Inode 3
Root inode
Step 6. Fetch Data block 253.
File Attributes
253
Direct block 1
inode 3

60. Inode 0 Inode 1 Inode 2 Inode 3
File Attributes
100
400
File Attributes
Direct block 0
Direct block 1
File Attributes
180
Direct blbock 1
File Attributes
253
Direct block 1
Inode Inode Inode Inode 3
Root inode
Step 6. Fetch Data block 253.
Orc
Poo
wimp
Data block 253. Directory listing for /usr/pmd

61. The steps in looking up /usr/ast/mbox
The UNIX File System
The steps in looking up /usr/ast/mbox

62. Exam
Comprehensive
Will cover essentially the same concepts from previous tests.
Will cover virtual memory and Unix file system organization and searching.
VM:
Understand page replacement algorithms.
What they are based on.
Work through optimal, LRU, clock, counter as done in class.

63. Understand demand paging.
Page faults. What causes a page fault and what happens!
When exam asks to solve a problem using approach shown in class, please do so.