Chapter 11 FileSystem Interface

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Chapter 11 File-System Interface
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Outline

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

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11.1 File Concept
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Introduction

• OS abstracts from the physical storage devices to
  define a logical storage unit File
• Types
• Data numeric, alphabetic, alphanumeric, binary
• Program source and object form

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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 (and the maximum allowable)
  size
• Protection controls who can read, write,
  execute
• 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 physical
  storage device.

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File Operations

• Delete
• Search the directory
• Release all file space
• Truncate --
• All attributes to remain unchanged, but for the
  file to be reset to length zero
• Open(Fi)
• Search the directory structure on disk for Fi
• Move the content of the directory entry to memory
• Close(Fi)
• move the content of entry Fi in memory to
  directory structure on disk

• Create
• Allocate space
• Make an entry in the directory
• Write
• Search the directory
• Write is to take place at the location of the
  write pointer
• Read
• Search the directory
• Read is to take place at the location of the read
  pointer
• Reposition within file file seek
• Set the current file pointer to a given value

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Open-File Table

• OS keeps a small table containing information
  about all open files
• Each time a file is opened, the content of the
  directory entry of the file is moved into the
  table
• File Handle (file descriptor, file control
  block) an index into the table for a file
• Two levels of internal tables in multi-user OS
• Per-process table All the files opened by a
  process
• Current file pointer
• System-wide open-file table Process independent
  information
• Location of file on disk, access dates, file size
• File open count

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A Typical Open-File Table
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Two-Level Open-File Table
Open File Table for Proc. A
System-Wide Open File Table
Open File Table for Proc. B
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File Types Name, Extension
If OS recognizes the type of a file, it can
operate on the file in a reasonable way
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File Structure

• None - sequence of words, bytes
• Simple record structure
• Lines
• Fixed length
• Variable length
• Complex Structures
• Formatted document
• Relocatable load file
• All operating systems must support at least one
  structure that of an executable file
• Who decides
• Operating system
• Program

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11.2 Access Methods
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Access Methods

• Sequential Access information is processed in
  order
• read next
• write next (append to the end of the file)
• reset to the beginning of file
• skip forward or backward n records
• Direct Access a file is made up of fixed length
  logical blocks or records
• read n
• write n
• position to n
• read next
• write next
• rewrite n

Sequential-access file
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Access Methods Index and Relative File
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11.3 Directory Structure
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Directory Structure

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

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A Typical File-System Organization
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Information in a Device Directory

• Name
• Type file, directory, symbolic link, special
  file
• Address device blocks to store a file
• Current length
• Maximum length
• Date last accessed (for archival)
• Date last updated (for dump)
• Owner ID (who pays)
• Protection information (discuss later)

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Operations Performed on Directory

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

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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 Pascal programs, all
  games, )

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Single-Level Directory

• A single directory for all users.

• Naming problem
• Grouping problem

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Two-Level Directory

• Separate directory for each user.

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

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Tree-Structured Directories
Subdirectory is just a special type of file
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Tree-Structured Directories (Cont.)

• Efficient searching
• Grouping Capability
• Current directory (working directory)
• cd /spell/mail/prog
• cat list
• Create a new file/subdirectory is done in current
  directory
• Absolute path name relative path name
• How to handle the deletion of a directory

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Acyclic-Graph Directories

• Can have shared subdirectories and files.
• A shared file can have different names (aliasing)
• Shared file implementation
• Symbolic link a pointer to another file or
  directory
• ln -s /spell/count /dict/count
• Hard link duplicate all information (especially
  device address) in shared directory

/spell/count
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Symbolic VS. Hard Link
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Acyclic-Graph Directories (Cont.)

• If /dict/w/list is deleted ? dangling pointer.
• Solutions Preserve the file until all references
  to it are deleted
• Backpointers a list of all references to a file
  (directory entries or symbolic links)
• Variable size records a problem
• Entry-hold-count solution a count
• A count of the number of references
• Used in UNIX for hard links

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General Graph Directory
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General Graph Directory (Cont.)

• 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
  detectionalgorithm to determine whether it is OK
• Traversal problem
• Dont traverse the same directory or file twice

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11.4 File-System Mounting
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File-System Mounting

• A file system must be mounted before it can be
  accessed
• A file system is mounted at a directory called
  the mount point
• Mounting steps mount /device/dsk /users
• Give the device name and the mount point
• Verify that the device contains a valid file
  system
• Check if the device directory has a valid format
• OS notes that a file system is mounted at the
  specified mount point
• What if the mount-point contains files?
• The original files are not available, until the
  file system is un-mounted
• Same file system can be mounted repeatedly, at
  different mount points

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File-System Mounting
Existing
Mount Point
Un-mounted Partition(/device/disk)
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11.5 File Sharing
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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
• A remote file system must be mounted before using
• mount claven.lib.nctu.edu.tw/OS /chow/book
• Once files are mounted, they are accessed by
  using the concatenated logical path names without
  referencing either the remote hosts or local
  devices

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Remote File-System Mounting Example
root
root
Export
chow
OS
Mount
paper
book
DFS
DSM
/OS/DSM
Local Client
Remote Server
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Remote File-System Mounting

• Different clients may perceive a different FS
  view
• To achieve a global FS view SA enforces
  mounting rules
• Export a file server restricts/allows the
  mounting of all or parts of its file system to a
  predefined set of hosts
• The information is kept in the servers export
  file
• File system mounting
• Explicit mounting clients make explicit mounting
  system calls whenever one is desired
• Boot mounting a set of file servers is
  prescribed and all mountings are performed the
  clients boot time
• Auto-mounting mounting of the servers is
  implicitly done on demand when a file is first
  opened by a client

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No Global Naming in NFS
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A Simple Automounter for NFS
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11.6 Protection
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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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Access Lists and Groups

• Mode of access read, write, execute
• Three classes of users owner, group, universe
• User name and user identifier
• Group name and group identifier
• 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
• In UNIX three field of 3 bits each
• field read, write, execute
• Bits r(4), w(2), x(1)

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Access Lists and Groups (Cont.)