1. File system(conti..)
Lecture 24
14 November 2018

2. File Sharing Sharing of files on multi-user systems is desirable.
Sharing may be done through
Duplicating files
Common login for members of a team
Setting appropriate access permissions
Common groups for members of a team
Links
14 November 2018

3. Protection File owner/creator should be able to control
What can be done
By whom
Types of access
Read
Write
Execute
Append
Delete
List
14 November 2018

4. UNIX Protection Mode of access: read, write, and execute (r, w, x)
Three classes of users: owner, group, and others
r w x
a) Owner access: 7  b) Group access: 6  1 1 0
c) Public access: 1  0 0 1
14 November 2018

5. UNIX Protection
Ask system administrator to create a group and add some users to the group
For a particular file or subdirectory, set appropriate access permissions for proper protection
14 November 2018

6. Default Permissions 777 for executable files and directories
666 for text files
Can be changed with the umask command
14 November 2018

7. UNIX Protection chmod 761 game1 owner group others Read Write Execute
14 November 2018

8. UNIX Protection chmod 755 projectAthena owner group others
Read Write Search
Read Search
Read Search
14 November 2018

9. UNIX Protection Sample commands chmod 700 ~ chmod 744 ~/file
chmod 755 ~/directory
ls –l ~
ls –ld ~
ls –l prog1.c
ls –ld ~/courses
14 November 2018

10. UNIX Protection The umask Command
Sets default permissions on newly created files and directories as (default permissions – mask value)
umask
umask 022
touch temp1
ls –l temp1
14 November 2018

11. File Control Block
UNIX inode
14 November 2018

12. In-Memory Data Structures
The following upper-level data structures needed for file system support.
Mapping directory entry for the file to the file control block at the time of opening a file.
Mapping used while reading or writing a file.
14 November 2018

13. In-Memory Data Structures
14 November 2018

14. Per Process File Descriptor Table
File Descriptor to File Contents
Per Process File Descriptor Table
File Table
Inode Table
File Descriptor 1 2 3 4
OPEN_MAX — 1
File’s contents … … … … …
14 November 2018

15. Space Allocation Methods
Contiguous allocation
Linked allocation
Indexed allocation
14 November 2018

16. Contiguous Allocation
Each file occupies a set of contiguous blocks on the disk
Best-fit, first-fit, or worst-fit algorithm
Directory entry contains starting block number and file size (in blocks)
Good sequential and random access
External fragmentation
User needs to declare file size
Expensive file growth
14 November 2018

17. Contiguous Allocation
14 November 2018

18. Linked Allocation
Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.
Simple – need only starting address
No wastage of space
No random access
14 November 2018

19. Linked Allocation
14 November 2018

20. Index Allocation Brings all pointers together into the index block
Logical view
Index
table
Data
blocks
14 November 2018

21. Index Allocation
14 November 2018