Computer File Sharing Systems

Operating Systems

• Operating systems manage free space in the file system using various techniques, including the free-list approach and the counting approach.

Grouping

• A modification of the free-list approach stores addresses of n free blocks in the first free block, with the last block containing addresses of another n free blocks.
• This approach allows for quick retrieval of addresses of a large number of free blocks.

Counting

• The counting approach keeps track of the address of the first free block and the number (n) of free contiguous blocks that follow the first block.
• Each entry in the free-space list consists of a disk address and a count, and can be stored in a balanced tree for efficient lookup, insertion, and deletion.

File Management

• File management involves file sharing, access control, and protection schemes to ensure secure access to files.

File Sharing

• File sharing is desirable in multi-user systems and can be implemented through protection schemes.
• On distributed systems, files can be shared across a network using methods like Network File System (NFS).
• User IDs, group IDs, and owner permissions determine file access rights.

Remote File Systems

• Remote file systems allow file system access between systems, either manually via programs like FTP or automatically using distributed file systems.
• The World Wide Web is another way to access remote files semi-automatically.

Protection

• File owners should be able to control what can be done to their files and by whom.
• Types of access include read, write, execute, append, delete, and list.

Access Lists and Groups

• Unix/Linux systems have three classes of users: owner, group, and public.
• Access modes include read, write, and execute (RWX).
• Users can be grouped together using commands like chgrp.

Windows Access-Control List Management

• Windows uses Access-Control Lists (ACLs) to manage file access.

Files and Directories - System Calls

• System calls for file management include create, delete, open, close, read, write, and seek.

I/O Management, System Protection, and Security

• I/O management involves managing input/output operations between devices and programs.
• System protection involves achieving security goals like confidentiality, integrity, and availability.

System Protection - Goals, Principles, and Domain of Protection

• System protection aims to prevent unauthorized access, use, disclosure, modification, or destruction of resources.
• Principles of protection include least privilege, separation of privilege, and accountability.
• The domain of protection includes hardware, software, and human elements.

SCAN Algorithm

• The SCAN algorithm (also called the elevator algorithm) schedules disk I/O requests to minimize head movement.
• The algorithm moves the head from one end of the disk to the other, servicing requests as it goes.

C-SCAN Algorithm

• The C-SCAN algorithm is a variant of SCAN that provides a more uniform wait time.
• The head moves from one end of the disk to the other, servicing requests, and then immediately returns to the beginning of the disk without servicing any requests on the return trip.
• C-SCAN treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

Learn about the concepts and methods of file sharing in multi-user and distributed systems, including remote file systems and networking protocols.