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Chapter: Operating Systems : File Systems

Directory Structure

There are five directory structures. They are Single-level directory, Two-level directory, Tree-Structured directory, Acyclic Graph directory, General Graph directory.



There are five directory structures. They are


        Single-level directory


        Two-level directory


        Tree-Structured directory


        Acyclic Graph directory


        General Graph directory


1. Single Level Directory


The simplest directory structure is the single- level directory.

All files are contained in the same directory.



When the number of files increases or when the system has more than one user, since all files are in the same directory, they must have unique names.



2. Two – Level Directory

 In the two level directory structures, each user has her own user file directory (UFD).

 When a user job starts or a user logs in, the system’s master file directory (MFD) is searched. The MFD is indexed by user name or account number, and each entry points to the UFD for that user.

 When a user refers to a particular file, only his own UFD is searched.

 Thus, different users may have files with the same name.

 Although the two – level directory structure solves the name-collision problem



v Users cannot create their own sub-directories.




3. Tree Structured Directory


v A tree is the most common directory structure.


v The tree has a root directory. Every file in the system has a unique path name.


v A path name is the path from the root, through all the subdirectories to a specified file.


v A directory (or sub directory) contains a set of files or sub directories.


v A directory is simply another file. But it is treated in a special way.


v All directories have the same internal format.


v One bit  in each directory entry defines the entry as a file (0) or as  a subdirectory (1).


v Special system calls are used to create and delete directories.


v Path names can be of two types: absolute path names or relative path names.


v An absolute path name begins at the root and follows a path down to the specified file, giving the directory names on the path.


v A relative path name defines a path from the current directory.



4. Acyclic Graph Directory.


An acyclic graph is a graph with no cycles.


To implement shared files and subdirectories this directory structure is used.


An acyclic graph directory structure is more flexible than is a simple tree structure, but it is also more complex. In a system where sharing is implemented by symbolic link, this situation is somewhat easier to handle. The deletion of a link does not need to affect the original file; only the link is removed.


Another approach to deletion is to preserve the file until all references to it  are  deleted.


To  implement  this  approach,  we  must  have  some mechanism  for  determining  that

the last  reference to the file has been deleted.

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