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File Sharing

File sharing means that multiple users can access the same file at the same time, while the system only retains one copy of the file. This method not only saves auxiliary storage and main memory space, but also reduces the number of I/O operations, improving user experience and system resource utilization.

Implementation methods of file sharing

File sharing is usually implemented using the following methods:

Directory link method is in a tree directory structure, when multiple users need to access the same file, a directory entry (i.e. link) is created for the shared file in their respective user file directory tables. The directory entry stores a pointer to the shared file.

Features:

  • Tree directory becomes a mesh directory: Multiple users' directory entries can point to the same shared file.

  • Search path is not unique: Users can access shared files through different paths.

  • Possible problems:

  • Delete exception: When a user deletes his own directory entry, the file is not actually deleted unless all users delete the directory entry.

  • Update exception: After a shared file is modified by any user, the content accessed by all users will change.

Index node link method is to achieve sharing through index node in the file system. Each file has an index node, which contains the file's metadata (such as storage address, file length, etc.). The directory entry only stores the file name and the pointer to the file's index node.

Sharing implementation:

  • The directory entries of the shared user point to the same index node.
  • Link counter (count): Each index node has a counter that records the number of directory entries currently linked to the node.
  • Delete operation: When the user deletes a file, only its directory entry is deleted and the index node counter is reduced; only when the counter is 0, the actual data of the file will be deleted.

Advantages:

  • No need to copy the file content when sharing files, saving storage space.
  • Unified management of file metadata for easy maintenance and modification.

Symbolic link method is to achieve sharing by creating a symbolic link file (Symbolic Link). In this method, the system call process link is used to create a new symbolic link file for the shared file and record it in the user's directory entry. The symbolic link file contains the path of the target file.

Advantages:

  • Avoid dangling pointers: The symbolic link records the path of the target file, not the pointer of the actual data block of the file, so there will be no dangling pointer problem.

  • Cross-file system sharing: Symbolic links can realize arbitrary file sharing in a network environment.

Disadvantages:

  • Increased system overhead: The system needs to manage symbolic link files and their path information.

  • Occupy auxiliary storage space: Symbolic link files will occupy a small amount of auxiliary storage space.

Comparison of various file sharing methods

Method Advantages Disadvantages
Directory link method No need to copy shared files, saving storage space Directory structure becomes mesh, paths are not unique, prone to exceptions
Index node link method Unified management of metadata, link counters to avoid accidental deletion of files Limited to the same file system, cannot be shared across file systems
Symbolic link method Sharing across file systems to avoid dangling pointers Increase system overhead and occupy auxiliary storage space