Chapter 10: FileSystem Interface

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Chapter 10 File-System Interface
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Motivation

• What is a file system?
• Why necessary?
• How to unify multiple file systems?

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What is a file system?
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File Concept

• Contiguous logical storage space
• With structure defined by OS/application
• Simple record structure
• Lines
• Fixed length
• Variable length
• Complex Structures
• Formatted document

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File Attributes

• Name for human identification
• Identifier unique tag (number) within file
  system
• Type
• Location
• Size
• Protection controls who can do reading,
  writing, executing
• Time, date, and owner data for protection,
  security, and usage monitoring
• File information kept in directory structure

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File Operations

• Create
• Write
• Read
• Reposition within file (eg., file seek)
• Delete
• Truncate (erasing the contents)

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Implementing File Operations
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Open Files

• Necessary information
• 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 (eg., multiple processes)
• Disk location of the file cache of data access
  information
• Access rights per-process access mode information

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Open File Locking

• Provided by OS/file system
• Mediates access to a file
• Mandatory or advisory
• Mandatory access is denied depending on locks
• Advisory processes can find status of locks and
  decide what to do (eg., readonly)

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File Locking Example

• 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("fi
  le.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()

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File Locking Example

• // this locks the second half of the file -
  shared
• sharedLock ch.lock(raf.length()/21,
  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()

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File Types Name, Extension
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Access Methods?

• File Attribute/Operation/Type
• Access?

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Access Methods

• Sequential Access
• read next
• write next
• reset
• Direct Access
• read n
• write n
• position to n
• n relative block address
• Indexed Access
• read (smith)

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Remote Control Analogy
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Sequential vs. Direct Access
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Brainstorming

• What does it take to support a direct access?
• Contiguous
• Otherwise
• What does it take to support an indexed access?

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Indexed Access
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Directory

• File
• Directory
• Why? What? How?

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Why Necessary?

• Efficiency locating a file quickly
• Convenience
• Naming convenient to users
• Files w/ same name can exist for different
  directories
• Grouping logical grouping of files by
  properties, (e.g., all Java programs, all games,
  )

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Directory Structure

• A collection of nodes containing information
  about files within

Directory
Files
F 1
F 2
F 3
F 4
F n
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Directory Operations

• Create/delete/rename directories
• List a directory
• Search for a file
• Create a file
• Delete a file
• Rename a file
• Traverse the file system (eg., backup)

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What is an ideal structure?
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Single-Level Directory

• A single directory for all users

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Two-Level Directory

• Separate directory for each user

• Can have the same file name (if by different
  user)
• Inefficient search
• No grouping capability

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Then what?
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Tree-Structured Directories
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Tree-Structured Directories (Contd)

• Efficient searching
• Grouping capability
• Locating files
• Absolute path (from root)
• Relative path (from current)

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Is the structure always a tree?
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Acyclic-Graph Directory Structure

• May have shared subdirectories and files

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Pros and Cons

• Avoid inconsistent copies
• Complication

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Acyclic-Graph Complications

• dangling pointer
• Solutions
• Whenever file is deleted (soft link)
• Wait until all refs deleted (hard link)
• How can you implement these solutions?

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Now, General Graph Directory
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Brainstorming

• What are new complications?
• What are the solutions?

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General Graph Directory

• New complications
• Traversal may take forever
• ? possibly infinite paths
• Deciding whether to delete
• ? Reference count is infinite

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Unified file system structure

• Multiple disks
• Multiple machines

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NN mapping
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File System Mounting

• A remote file system can be mounted

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Do you see potential problems?
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Case Study Remote Failures

• Remote file systems add new failure modes, (e.g.,
  network failure, server failure
• Recovery from failure can involve state
  information
• Stateless protocols such as NFS include all
  information in each request, allowing easy
  recovery but less security

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File Consistency
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Consistency Unix vs. AFS

• Consistency semantics specify how multiple users
  are to access a shared file
• Unix file system implements
• Writes to an open file visible immediately to
  other users of the same open file (modified,
  reload?)
• AFS has session semantics
• Writes only visible to sessions starting after
  the file is closed

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File Protection
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A Sample UNIX Directory Listing
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Access Lists and Groups

• Mode of access read, write, execute
• RWX
• a) owner access 7 ? 1 1 1 RWX
• b) group access 6 ? 1 1 0
• RWX
• c) public access 1 ? 0 0 1
• For a particular file (say exam_solution) or
  subdirectory, define an appropriate access.

owner
group
public
exam_solution
chmod
761
Attach a group to a file chgrp
staff exam_solution
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Protection

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

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Windows Example
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End of Chapter 10