Chapter 11: FileSystem Interface

1
Chapter 11 File-System Interface

• File Concept
• Access Methods
• Directory Structure
• Protection
• File-System Structure
• Allocation Methods
• Free-Space Management
• Directory Implementation
• Efficiency and Performance
• Recovery

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File Concept

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

3
File Structure

• None - sequence of words, bytes , interpretation
  dependent upon creation
• Simple record structure
• Lines (EOL marker)
• Fixed length (all fields same length or known
  pattern)
• Variable length (self-describing data sequence
  of fields in the form of sequence of pairs (byte
  count, indicated number of bytes). Fields can be
  nested inside of field data.)
• Complex Structures
• Formatted document divided into sections, each
  with header describing format of section,
  possibly with embedded codes as well.
• Relocatable load file
• Can simulate last two with first method by
  inserting appropriate control characters.
• Who decides
• Operating system Must manipulate the files so
  may enforce that all files have some recognizable
  structure. Must differentiate executable files
  from others.
• Program creates files and is responsible for
  processing them.

4
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, 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.

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File Operations

• Create
• Write
• Read
• Reposition within file file seek
• 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.

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File Types Name, Extension
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Access Methods

• Sequential Access information in the file is
  processed in order, one record after the other.
• read next
• write next
• reset
• no read after last write
• (rewrite)
• Direct Access information in the file is
  accessed based on the logical records/block.
• read n
• write n
• position to n
• read next
• write next
• rewrite n
• n relative block number

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Sequential-access File
9
Simulation of Sequential Access on a
Direct-access File
cp a variable to record the current access
position
10
Index-based Access Methods

• Index which contains pointers to the various
  blocks is used for fast access information from a
  file.

Example of Index and Relative Files
11
Directory Structure

• A collection of nodes containing information
  about all files.

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

• A file system is broken into partitions. A
  partition is a low-level structure in which files
  and the directories reside.
• Some systems use partitions to provide several
  separate areas with on e disk, treating each as a
  separate storage device, whereas others allow
  partitions to be larger than a disk so that they
  can group disks into one logical structure.
• Each partition contains information about files
  within it. This information is kept in entries in
  a device directory.
• Name
• Type
• Address
• 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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A Typical File-system Organization (Cont.)
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Operations Performed on Directory

• Search for a file find the entry for a
  particular file or files with similar names.
• Create a file create new files and add them to
  the dir.
• Delete a file remove a file from the dir.
• List a directory list the files in a dir.
• Rename a file change the name of a file.
• Traverse the file system access every dir., and
  every file within a dir. structure.

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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 Java 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
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Tree-Structured Directories (Cont.)

• Efficient searching
• Grouping Capability
• Current directory (working directory)
• cd /spell/mail/prog
• type list

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Tree-Structured Directories (Cont.)

• Absolute path name full path name from root to
  specified file.
• relative path name defines a path from the
  current dir.
• 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

mail
prog
copy
prt
exp
count
Deleting mail ? deleting the entire subtree
rooted by mail.
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Acyclic-Graph Directories

• Have shared subdirectories and files.

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Acyclic-Graph Directories (Cont.)

• 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.

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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.