File Systems - III

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File Systems - III
CS423UG Operating Systems

• Indranil Gupta
• Lecture 24
• 10/21/2005

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ls al (or dir)

• Directories and their organization

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Directory Structure Organization

• Maps symbolic filenames into disk locations,
  resolves entire pathnames
• Operations on a directory (note difference from
  file operations)
• list directory
• search for a file
• create file
• backup, archival, file migration

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Directory Operations

• Readdir
• Rename
• Link
• Unlink

1. Create
2. Delete
3. Opendir
4. Closedir

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Directory Contents

• For each file/subdir
• file name symbolic name
• file type indicates format of file
• location device and location
• usage count (how many processes have this file
  open)
• size
• protection
• creation, access, and modification date
• owner identification

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Single Level (Flat) Directory
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Problems With Single Level Directory

• more than one user?
• large file systems ?
• moving files from one system to another?
• name clashes?
• modularity?

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Two-level Directory

• Introduced to remove naming problems between
  users
• First level contains list of user directories
• Second level contains user files
• System files kept in separate directory or level
  1
• Sharing accomplished by simply providing to F.S.
  other users id filename

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Two-level Directory
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Tree Structured (Hierarchical) Directories

• Arbitrary depth of directories
• Each directory contains further subdirectories
  and files
• Leaf nodes are files
• Interior nodes are directories
• Path name lists nodes to traverse to find node
• Use absolute paths from root
• Use relative paths from current working directory
  for the calling process

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Tree Structured Directories
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Acyclic-Graph Structured Directories

• Allow directories to link to one another, allow
  multiple directories to contain same file (how?)
• Results in Acyclic graphs
• Two users can name same file or same directory
• Duplicate paths may complicate task of backing up
  periodically

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Acyclic-Graph Structured Directories
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General Graph Structured Directories

• Allow cycles
• More flexible
• More costly
• Need garbage collection (circular structures)
• There could be a subset of directories all with
  reference count gt 0, but none of these are
  reachable from the root
• When doing recursive search for a file within the
  file system, must prevent infinite searches

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General Graph Structured Directories
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Path Names
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Announcements

• Reading for this lecture 6.2
• Reading for next lecture
• Next week
• I am out of town next week M and W
• Monday will have lecture on File systems (given
  by Ramses Morales, PhD student in CS)
• Wednesday !In-class quiz!