l5

"/home/yossef/notes/Su/os2/l5.md"

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- **fileName**: l5
- **Created on**: 2026-01-04 21:22:39

This explanation covers the full lecture on File-System
Interface from the sources, using simple language to make
it easy to understand.

A file is a way to store information in a "contiguous logical
address space". This means the computer sees it as one
continuous piece of data.

Types of Files: They can be Data (numbers, text, or
binary) or Programs.
Structure: Some files are just a long string of bytes.
Others have a "record structure" with fixed or variable
lengths. The Operating System (OS) or the Program
itself decides this structure.
Attributes: Every file has details like a Name (for
humans), a Unique Identifier (a number for the system),
Type, Location, Size, and Protection settings.

The OS performs several tasks on files:

Basic Tasks: Create, Read, Write, Delete, and
Truncate (clearing file content but keeping the file).
Open and Close: When you Open a file, the system
moves its info from the disk to the memory. When you
Close it, the info moves back to the disk.
Managing Open Files: The system tracks a File
Pointer (where you last read or wrote), an Open Count
(how many users are using it), and your Access Rights.
File Locking: To prevent errors when two users access
one file, the OS uses Locks.
- Exclusive Lock: Only one user can write.
- Shared Lock: Many users can read at the same time.
- Mandatory vs. Advisory: Mandatory locks are enforced
  by the system. Advisory locks let programs decide what
  to do after checking the lock status.

Disks and Partitions: A disk can be divided into
Partitions (also called minidisks or slices). A
partition that holds a file system is called a Volume.
Directories: A directory is like a map that contains
nodes pointing to all the files.
Directory Tasks: You can search for files, create or
delete them, rename them, and traverse (walk through)
the entire file system.

The sources describe different ways to organize directories:

Single-Level: One big folder for everyone. This makes
naming and grouping files difficult.
Two-Level: Each user gets their own folder. You use
Path Names to locate files.
Tree-Structured: The most common type. Folders can
have subfolders. You can use Absolute paths (from the
root) or Relative paths (from the current location).
Acyclic-Graph: Allows sharing folders or files. One
file can have multiple names (Aliasing).
General Graph: Allows complex links, but care must be
taken to avoid Cycles (endless loops).

Mounting: A file system must be Mounted (attached)
to a specific mount point before it can be accessed.
Sharing: In multi-user systems, User IDs and
Group IDs control who can access files.
Remote Systems: Files can be shared over networks
using protocols like NFS (Unix) or CIFS (Windows).
Failures: Remote access may fail if the server or
network goes down. Stateless protocols (like NFS)
include all needed info in each request, making recovery
easier.

Consistency Semantics: Defines how file changes are
seen by users. In Unix, changes are visible
immediately. In AFS, changes appear only after the
file is closed.
Protection: The file owner controls permissions such
as Read (R), Write (W), Execute (X), Append,
Delete, or List.
Access Classes:
- Owner: The creator of the file.
- Group: A specific set of users.
- Public: All other users.
Example: In Unix, the command chmod 761 game gives
the owner full access (7), the group read/write (6), and
the public execute-only access (1).

Think of the File System as a huge library.

Files are the books.
Attributes are the labels on the book spine.
Directories are the card catalogs showing where books
are stored.
Mounting is opening a new wing of the library.
Protection is the library card that decides whether
you can read a book or write in it.

mind map

images/15_mind_map.png|500

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