CHAPTER 11: FILE-SYSTEM INTERFACE

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CHAPTER 11 FILE-SYSTEM INTERFACE

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

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FILE CONCEPT

• Introduction
• File attributes
• File types
• File structures
• File operations

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File concept

• What is a file
• A file is named collection of related information
  that is recorded on secondary storage.
• Content types
• Data
• numeric
• alphabetic
• alphanumeric
• binary
• Program
• source programs
• Object forms

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File concept Attributes

• File attributes
• Name The symbolic file name in human-readable
  form
• Identifier to identify the file within the file
  system (Internal)
• Type to support different types.
• Location the location on the secondary storage
  device.
• Size the current size (and/or its maximum size)
• Protection who can do reading, writing,
  executing, and so on.
• Time, date, and user identification This
  information may be kept for creation, last
  modification, and usage monitoring.
• Typically, the directory entry consists of the
  files name and its unique identifier. The
  identifier in turn locates the other file
  attributes. One of the file attributes is the
  location of file.

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File concept Types

• If an OS recognizes the type of a file, it can
  then operate on the file in reasonable ways.
• Some common file types (See the next slide)
• The TOPS-20 OS If the user tries to execute an
  object program whose source file has been
  modified (or edited) since the object file was
  produced, the source file will be recompiled
  automatically.
• The Apple Macintosh OS Each file has a type and
  also has a creator attribute containing the name
  of the program that created it.
• The Linux File types are just hints. How to
  determine a file type on the Linux.

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File concept Types
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File concept Structure

• File structures
• None - sequence of words, bytes
• Simple record structure
• Lines, Fixed length, Variable length
• Complex Structures
• Formatted document
• Relocatable load file
• Can be implemented by inserting appropriate
  control characters.
• Who decides Operating system and Program
• Conclusion Too few structures make programming
  inconvenient, whereas too many cause OS bloat and
  programmer confusion.

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File concept Operations

• Create to allocate the file space and allocate
  the directory entry
• Read/Write/Seek to find the directory entry and
  keep a read/write pointer or seek to a new
  position.
• Delete to find the directory entry and release
  all file space and erase the directory entry
• Truncate to find the directory entry and release
  some file space
• Open(F) to search the directory structure on
  disk for entry F, and copy the directory entry
  into the open-file table and allocate a file
  descriptor.
• Close (F) to copy the directory entry in the
  open-file table to the directory structure on
  disk and free the file descriptor

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File concept Operations

• File tables
• Single user Per-process file table ? secondary
  storage
• Multiple-users Per-process file table ?
  system-wide table ? secondary storage
• Several pieces of information are associated with
  an open file
• File pointer
• File open count
• Disk location of the file
• Access rights

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FILE ACCESS

• Sequential Access
• read next
• write next
• reset
• Direct Access
• read n
• write/rewrite n
• position to n
• read next
• write next
• n relative block number
• To simulate sequential access by direct access is
  easy
• (To simulate direct access by sequential access
  is NOT easy)

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File access Sequential-access
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File access Direct-access
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File access Other access

• Index file
• Master index
• ? index file
• ? file data
• (ISAM Indexed Sequential-Access Method from IBM)

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File access Other access
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DIRECTORY STRUCTURE

• Disks ? partitions/minidisks/volumes ?FS ?
  Directories ?Files
• The directory is used to organize files (and
  directories)
• The directory can be viewed as a symbol table
  that translates file names into their directory
  entries.
• The operations on directories
• Search a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

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Directory structure
A Typical File-system Organization
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Directory structure Criteria

• 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
• all Java programs
• all games

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Directory structures

• Single-level directory
• Two-level directory
• Tree-structured directory
• Acyclic graph directory
• General graph directory

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Directory structure Single-Level Directory

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

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Directory structure Two-Level Directory

• Separate directory for each user.
• MFD/UFD/FileName (Path name)
• Can have the same file name for different user
• Efficient searching
• No grouping capability

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Directory structure Tree-Structured Directories
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Directory structure 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 ltfile-namegt
• Creating a new subdirectory is done in current
  directory.
• mkdir ltdir-namegt
• Example if in current directory /mail
• mkdir count

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Directory structure Acyclic-graph directories
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Directory structure Acyclic-graph directories

• Tree-structured directory sharing ?
  Acyclic-Graph Directories
• Linux ln
• hard link
• soft link
• Potential problems
• To traverse the FS
• To delete a file (for symbolic link and hard link)

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Directory structure General graph directory
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Directory structure General graph directory

• Potential problems
• To traverse the FS (maybe looping forever)
• To determine when a file can be deleted.
• Garbage collection

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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.

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(a) Existing. (b) Unmounted Partition
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File System Mounting

• /etc/fstab
• Device
• Mount point
• File system type
• Mount options
• For dump
• For fsck
• Mount/umount, swapon/swapoff
• /proc

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FILE SHARING

• For multi-user OS
• Sharing of files on multi-user systems is
  desirable.
• Sharing may be done through a protection scheme.
• File sharing over the network
• FTP
• WWW
• NFS

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File Sharing Remote file systems

• The C/S Model
• The server
• To declare that a resource is available to
  clients
• To specify exactly which resource and exactly
  which clients.
• The client
• To access the remote files.
• The protocol
• Authentication
• Operations.

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File Sharing Remote file systems

• How to use NFS
• Startup nfs
• /etc/rc.d/init.d/nfs
• setup or ntsysv
• /etc/exports
• Mount NFS
• mount server_machine/exported_fs mount_point
• /etc/fstab

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File Sharing Remote file systems

• Distributed Information Systems
• To provide a unified access to the information
  needed for remote computing.
• DNS domain name to IP address
• Yellow pages to centralize storage of user name,
  hostnames, printer information, and the like.
• Yellow pages ? NIS ? NIS (Sun Microsystems)
• Domains ? Active directory (Microsoft)
• ?LDAP (Lightweight directory-access protocol)

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File Sharing Remote file systems

• Failure Modes
• For one machine, to use RAID (Redundant Arrays of
  Inexpensive Disks) to prevent data loss
• For networked machines,
• Stateful mode
• Too many problems can cause operation to fail.
• not good enough.
• Stateless
• The protocol carries all the information needed
  to locate the appropriate file and perform the
  requested operation on a file.

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File Sharing Consistency semantics

• Consistency semantics
• specifies the the semantics of multiple users
  accessing a shared file simultaneously.
• In particular, these semantics should specify
  when modifications of data by one user are
  observable by other users.
• UNIX semantics
• Writes to an open file by a user are visible
  immediately to other users that have this file
  open at the same time
• One mode of sharing allows users to share the
  pointer of current location into the file

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File Sharing Consistency semantics

• AFS (Andrew File System) Semantics
• Writes to an open file by a user are not visible
  immediately to other users that have the same
  file open simultaneously.
• Once a file is closed, the changes made to it are
  visible only in sessions starting later. Already
  open instances of the file do not reflect these
  changes.
• Immutable-shared file semantics
• Once a file is declared as shared by its creator,
  it cannot be modified.
• An immutable file has two key properties. Its
  name may not be reused and its contents may not
  be altered.

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FILE PROTECTION

• File owner/creator should be able to control
• what can be done
• by whom
• No access ? some controlled access ? full access
• Types of access
• Read
• Write
• Execute
• Append
• Delete
• List

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File protection

• Access control lists
• Mode of access read, write, execute
• Three classes of users
• RWX
• a) owner access 7 ? 1 1 1 RWX
• b) group access 6 ? 1 1 0
• RWX
• 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.

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File protection

• Chmod
• Find
• Ls, cd, mkdir, cat, more, less, cmp, diff, sort,
  merge, tee, touch,

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Homework

• 3, 6, 7, 10, 11

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