[IOS] Ch 10 File-System

Ch 10 File-System

• 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 explore file-system protection
  

Ch 10.1 File Concept

• Contiguous logical address space
• Types:
  
  • Data
    
    • numeric
    • character
    • binary
  • Program

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

10.1.1 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 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

10.1.2 File Operations

• File is an abstract data type
• Create
• Write
• Read
• Reposition within file
• 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

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
  • 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

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

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();

            // 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)
                sharedLock.release();
        }
    }
}

Ch 10.2 Access Methods

• Sequential Access

    read next
    write next
    reset
    no read after last write
    (rewrite)

• Direct Access

    read n
    write n
    position to n
        read next
        write next
    rewrite n

• n = relative block number

Sequential-access File

Simulation of Sequential Access on Direct-access File

sequential access	implementation for direct access
reset	cp = 0;
read next	read cp; cp = cp + 1;
write next	write cp; cp = cp + 1;

Example of Index and Relative Files

Ch 10.3 Directory and Disk Structure

• Both the directory structure and the files reside on disk
• Backups of these two structures are kept on tapes

Disk Structure

• Disk can be subdivided into partitions
• Disks or partitions can be RAID protected against failure
• Disk or partition can be used raw – without a file system, or formatted with a file system
• Partitions also known as mini-disks, slices
• Entity containing file system known as a volume
• Each volume containing file system also tracks that file system’s info in device directory or volume table of contents
• As well as general-purpose file systems there are many special-purpose file systems, frequently all within the same operating system or computer

Operations Performed on Directory
- Search for a file

• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

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, …)

Single-Level Directory

• Single-Level Directory
  
  • Naming problem
  • Grouping problem

Two-Level Directory

• Separate directory for each user
• Path name
• Can have the same file name for different user
• Efficient searching
• No grouping capability

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>

• Creating a new subdirectory is done in current directory

    mkdir <dir-name>

Acyclic-Graph Directories

• 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
• New directory entry type
  
  • Link – another name (pointer) to an existing file
  • Resolve the link – follow pointer to locate the file

General Graph Directory

• 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

Ch 10.4 File System Mounting

• A file system must be mounted before it can be accessed
• A unmounted file system (i.e. Fig. 11-11(b)) is mounted at a
  mount point

Ch 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
• User IDs identify users, allowing permissions and protections to be per-user
• Group IDs allow users to be in groups, permitting group access rights

Remote File Systems

• Uses networking to allow file system access between systems
  
  • Manually via programs like FTP
  • Automatically, seamlessly using distributed file systems
  • Semi automatically via the world wide web
• Client-server model allows clients to mount remote file systems from servers
  
  • Server can serve multiple clients
  • Client and user-on-client identification is insecure or complicated
  • NFS is standard UNIX client-server file sharing protocol
  • CIFS is standard Windows protocol
  • Standard operating system file calls are translated into remote
    calls
• Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing
• Remote file systems add new failure modes, due to network failure,
  server failure
• Recovery from failure can involve state information about status of
  each remote request
• Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security
• Consistency semantics specify how multiple users are to access a shared file simultaneously
  
  • Similar to Ch 7 process synchronization algorithms
    
    • Tend to be less complex due to disk I/O and network latency (for remote file systems)
  • Andrew File System (AFS) implemented complex remote file sharing
    semantics
  • Unix file system (UFS) implements:
    
    • Writes to an open file visible immediately to other users of the same open file
    • Sharing file pointer to allow multiple users to read and write concurrently
  • AFS has session semantics
    
    • Writes only visible to sessions starting after the file is closed

Ch 10.6 Protection

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

Reference: Operating System Concepts 8th, by Silberschatz, Galvin, Gagne