1. Chapter 10: File-System 10.1 File Concept 10.2 Access Methods
10.3 Directory Structure
10.4 File-System Mounting
10.5 File Sharing
10.6 Protection

2. Objectives To explain the function of file systems
To describe the interfaces to file systems
To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures
To discuss the semantics of sharing files among multiple processes, users, and computers
To explore file-system protection

3. 10.1 File Concept
The operating system abstracts from the physical properties of its storage to define a logical storage unit, the file.
Files are mapped by the OS onto physical, usually nonvolatile, devices.
Use Ultra-Editor to examine contents of a file
File types:
Data, free form or formatted
numeric
character
binary
Program
source
object

4. File Structure None - sequence of words, bytes Simple record structure
Lines and Pages
Fixed length
Variable length
Complex Structures
Formatted document
Relocatable load file
Executable
Who decides:
Operating system
Program

5. File Attributes Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different file types
Location – pointer to the 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 secondary storage, like a disk

6. File Operations
File is an abstract data type with the following basic operations
create
write
The system must keep a writer pointer. File info in the directory also updated
read
The system must keep a read pointer.
reposition within file (known as file seek)
delete
truncate
Other operations
append, rename, copy, get/set file attributes

7. Open and Close Files
Most file operations involve searching the directory for a file
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
The OS normally maintains two-level open-file tables, per-process and system-wide

8. Open and Close 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
File-open 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

9. Open File Locking Provided by some operating systems and file systems
Mediates access to a file, like process synchronization
shared lock and exclusive lock
Mandatory or advisory:
Mandatory – access is denied depending on locks held and requested (Windows)
Advisory – processes can find status of locks and decide what to do (Unix)

10. File Locking Example – Java API
import java.io.*;
import java.nio.channels.*;
public class LockingExample {
public static final boolean EXCLUSIVE = false;
public static final boolean SHARED = true;
public static void main(String arsg[]) throws IOException {
FileLock sharedLock = null;
FileLock exclusiveLock = null;
try {
RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");
// get the channel for the file
FileChannel ch = raf.getChannel();
// this locks the first half of the file - exclusive
exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);
/** Now modify the data */
// release the lock
exclusiveLock.release();

11. File Locking Example – Java API (cont)
// this locks the second half of the file - shared
sharedLock = ch.lock(raf.length()/2+1, raf.length(), SHARED);
/** Now read the data */
// release the lock
sharedLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}finally {
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null) }

12. Common File Types
Unix: use magic number to indicate roughly file types
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13. Internal File Structure
All disks is performed in units of one block (physical record)
Logical records may vary in length
Packing a number of logical records into physical blocks is the common solution
Example: UNIX defines all files to be streams of bytes. Its logical record size is 1 byte.
Packing can be done either by user’s application or by the operating system
Internal fragmentation problem

14. 10.2 Access Methods Sequential Access (based on tape model) read next
write next
reset to the beginning
no read after last write
Direct Access (or relative access)
read n
write n
position to n
rewrite n
(n = relative block number)

15. Simulation of sequential access on a direct-access file
Some systems support only one of sequential access and direct access for files.
Simulation of sequential access on a direct-access file
Simulation of direct access on a sequential-access file is inefficient and clumsy

16. Example of Index and Relative Files
Other Access Methods
Example of Index and Relative Files

17. 10.3 Directory Structure
A disk may have several partitions. A partition may be with a file system. Several partitions, maybe from many disks, could form a volume that holds a file system.
A collection of nodes containing information about all files
F 1
F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes

18. A Typical File-system Organization

19. Operations Performed on Directory
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
for backup (to tape)

20. 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, …

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

22. Two-Level Directory Separate directory for each user
Can have the same file name for different user
Isolation or Allow access to other’s files?
If allowed, then use path name
Efficient searching
Use environment variable: search path
No grouping capability

23. Tree-Structured Directories

24. Tree-Structured Directories
Efficient searching
Grouping Capability
Current directory (working directory)
cd /spell/mail/prog
type list
Absolute or relative path name
Creating a new file is done in current directory
Delete a file
rm <file-name>
Delete a directory
MS-DOS will not delete a directory unless it is empty
Unix provides an option to delete all files and sub-directories under a directory

25. Tree-Structured Directories
Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if current directory is /mail
mkdir count
mail
prog
copy
prt
exp
count
In Unix “rm –f mail”  deleting the entire subtree rooted by “mail”

26. Acyclic-Graph Directories
Use link to have shared subdirectories and files
Another approach: duplicate all information about subdirectories and files in both sharing directories. But it is hard to maintain consistency when a shared file is modified.

27. Acyclic-Graph Directories
New directory entry type
Link – another name (pointer) to an existing file
Resolve the link – follow pointer to locate the file
Two different names (aliasing)
A file could have multiple absolute path names.
Traverse problem.
If dict deletes all  dangling pointer. Solutions:
Just wait for users to find out. It is used with symbolic links:
Preserve the file until all references to it are deleted. Unix uses this approach for hard links by keeping a reference count in the file information block.
Acyclic-graph could be maintained by prohibiting multiple references to directories
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28. 10.4 File System Mounting
A file system must be mounted before it can be accessed
A unmounted file system (i.e. Fig (b)) is mounted at a mount point
existing
unmounted volume
mount point

29. Mount Point
The OS is first given the name of the device and the mount point
The OS verifies that the device contains a valid file system
Read the device directory and verify the directory format
The OS notes in the directory structure that a file system is mounted at the specified mount point
If the volume is unmounted, the file system is restored to the situation before mounting
OS may impose semantics to clarify functionality
May disallow a mount over a directory containing files; or may obscure the directory’s existing files until the file system is unmounted
May allow the same file system to be mounted repeatedly, at different mount points; or it may allow only one mount per file system

30. Mount Examples
Macintosh searches for a file system on a disk first encountered. If found, the file system is auto-mounted at the root level
Windows OS maintains an extended two-level directory structure, with devices and volumes assigned drive letters.
Recent Windows allow a file system to be mounted anywhere in the directory tree
Windows auto-discover all devices and mount all located file systems at boot time
Unix has explicit mount commands

31. 10.5 File Sharing Sharing of files on multi-user systems is desirable
Sharing may be done through a protection scheme
On distributed systems, files may be shared across a network
Network File System (NFS) is a common distributed file-sharing method

32. File Sharing – Multiple Users
File sharing, file naming, and file protection are important in multiple-user systems
The system may allow a user to access other user’s files by default or it may require specific access grant
Most systems use the concept of file owner and group, as file attributes, to implement file sharing and protection
User IDs identify users, allowing permissions and protections to be per-user
Group IDs allow users to be in groups, permitting group access rights

33. File Sharing – Remote File Systems
Uses networking to allow file system access between systems
Manually via programs like FTP
Both anonymous and authenticated access
Automatically, seamlessly using distributed file systems, in which remote directories are visible from a local machine
Semi automatically via the world wide web, where a browser is needed to access remote files, and separate operations (a wrapper for ftp) are used to transfer files

34. The Client-Server Model
Client-server model allows clients to mount remote file systems from servers
Server can serve multiple clients
Client, specified by a network name or IP address, and user-on-client identification is insecure or complicated (by encryption)
NFS is standard UNIX client-server file sharing protocol
User’s ID on the client and server must match
Once the remote file system is mounted, file operation requests are sent on behalf of the user across the network to the server via the DFS protocol
Standard operating system file calls are translated into remote calls

35. Distributed Information Systems
Also known as distributed naming services
LDAP, DNS, NIS (network information service, yellow pages), Active Directory implement unified access to information needed for remote computing
In Windows CIFS (common internet file system), network information is used with user authentication to create a network login. A newer version is called active directory.
One distributed LDAP (lightweight directory-access protocol) could be used by an organization to store all user and resource information for all organization’s computers. The result is secure single sign-on for users.
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36. 10.6 Protection
Reliability is to keep the computer system from physical damage. (Chapter 12)
Protection is to keep it from improper access.
File owner/creator should be able to control:
what can be done
by whom
Basic types of controlled access
Read
Write
Execute
Append
Delete
List
Other high-level functions, like copying and
editing files may be implemented by making
lower-level system calls

37. Access Control Lists Mode of access: read, write, execute
Three classes of users
r w x
a) owner access 7  r w x
b) group access 6  1 1 0
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 a particular file (say game) or subdirectory, define an appropriate access.
owner
group
public
chmod
761
game
Attach a group to a file chgrp G game

38. Windows XP Access-control List Management

39. A Sample UNIX Directory Listing

40. Other Protection Approaches
Associate a password with each file
Disadvantages
The number of passwords that a user needs to remember
If only one password is used for all the files, then protection is on an all-or-none basis
Some system allow the user to associate a password with a directory
Adding protection mechanisms to single-user OS is difficult
Directory protection
Control the creation and deletion of files in a directory
Control whether a user could check the existence of a file in a directory. (Listing the contents of a directory)