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Directory Structures : Information 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)
Operations Performed on Directory
Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system
Organize the Directory (Logically) to Obtain
Efficiency – locating a file quickly. Naming – convenient to users. 1. Two users can have same name for different files. 2. The same file can have several different names. Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)
Single-Level Directory
A single directory for all users. Naming problem Grouping problem
Two-Level Directory Separate directory for each user.
•Path name •Can have the same file name for different user •Efficient searching •No grouping capability
Tree-Structured Directories
Efficient searching Grouping Capability Current directory (working directory) 1.cd /spell/mail / prog 2. type list 3. Absolute or relative path name 4. Creating a new file is done in current directory. 5. Delete a file rm <file-name> Creating a new subdirectory is done in current directory. mkdir <dir-name>
Acyclic-Graph Directories Have shared subdirectories and files.
Two different names (aliasing) If dict deletes list _ dangling pointer. Solutions:
Variable size records a problem. 1. Back pointers using a daisy chain organization. 2. Entry-hold-count solution.
General Graph Directory
How do we guarantee no cycles? 1. Allow only links to file not subdirectories. 2. Garbage collection. 3. Every time a new link is added use a cycle detection algorithm to determine whether it is OK.
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