File System Interface In Operating System Ppt/Pdf/Ebook Download

File System Interface

file system interface

  • File Concept
  • Access Methods
  • Directory Structure
  • File System Mounting
  • File Sharing
  • Protection

 

File Concept

  • Contiguous logical address space

 

Types:
  • Data
  1. numeric
  2. character
  3. binary

File Structure

 

  • None – sequence of words, bytes
  • Simple record structure
  1. Lines
  2. Fixed length
  3. Variable length
  • Complex Structures
  1. Formatted document
  2. Relocatable load file
  • Can simulate last two with first method by inserting appropriate control characters
  • Who decides:

File Attributes

Name: only information kept in human-readable form.
 
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, execution.

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

File Operations

Create
Write
Read
File seek – reposition within file
Delete
Truncate
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

Open Files:

Several pieces of data are needed to manage open files:
  1. File pointer:  pointer to last read/write location, per process that has the file open
  2. 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
  3. Disk location of the file: cache of data access information
  4. 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
  1. Mandatory – access is denied depending on locks held and requested
  2. 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
exclusiveLock.release();
} catch (java.io.IOException ioe) { 
System.err.println(ioe);
}finally { 
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null)
sharedLock.release();
}
}
}
 
Copy the above code

 

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
exclusiveLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}finally {
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null)
sharedLock.release();
}
}
}

 

File Types – Name, Extension

file type name extension

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:

Sequential access File

Simulation of Sequential Access on a Direct-access File

Example of Index and Relative Files

Example of Index and Relative Files

Directory Structure

A collection of nodes containing information about all files
Directory Structure
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes

A Typical File-system Organization

A Typical File system Organization

Information in a Device Directory

  • Name
  • Type
  • Address
  • Current length
  • Maximum length
  • Date last accessed (for archival)
  • Date last updated (for dump)
  • Owner ID
  • Protection information (discuss later)

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
  1. Two users can have same name for different files
  2. 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

A single directory for all users
A single directory
Naming
problem
Grouping
problem

Two-Level Directory

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

Tree-Structured Directories

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>
Example:  if in current directory   /mail
mkdir count
Tree Structured Directories
Deleting “mail” deleting the entire subtree rooted by “mail”

Acyclic-Graph Directories:

Have shared subdirectories and files
Acyclic Graph Directories

Solutions:

  • Backpointers, so we can delete all pointers Variable size records a problem
  • Backpointers using a daisy chain organization
  • Entry-hold-count solution

General Graph Directory

General Graph Directory
  • How do we guarantee no cycles?
  1. Allow only links to file not subdirectories
  2. Garbage collection
  3. Every time a new link is added use a cycle detection algorithm to determine whether it is OK

File System Mounting

  • A file system must be mounted before it can be accessed
  • A unmounted file system is mounted at a mount point
(a) Existing.  (b) Unmounted Partition
 
file system mounting
 

Mount Point

Mount Point
 
File Sharing
  1. Sharing of files on multi-user systems is desirable
  2. Sharing may be done through a protection scheme
  3. On distributed systems, files may be shared across a network
  4. Network File System (NFS) is a common distributed file-sharing method
File Sharing – Multiple Users
User IDs identify users, allowing permissions and protections to be per-user
Group IDs allow users to be in groups, permitting group access rights
File Sharing – Remote File Systems
  • Uses networking to allow file system access between systems
  1. Manually via programs like FTP
  2. Automatically, seamlessly using distributed file systems
  3. 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 implement unified access to information needed for remote computing

File Sharing – Failure Modes

  • 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

File Sharing – Consistency Semantics

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

Protection

File owner/creator should be able to control:
  • what can be done
  • by whom

Types of access Read

  1. Write
  2. Execute
  3. Append
  4. Delete
  5. List

Access Lists and Groups

Mode of access:  read, write, execute
Three classes of users
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.
access list and groups

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