Server Operating Systems

Role of Servers

• Server OS is a specialized OS designed to efficiently manage multiple users and high workloads.
• Examples of Server OS include Windows Server, Linux (Ubuntu Server, CentOS), and Unix-based servers.

Responsibilities of Server OS

• Process and Task Management: Handles multiple user requests simultaneously and prioritizes tasks for optimal performance.
• User Management: Supports multiple user accounts with different access privileges and implements authentication and security policies.
• Network and Remote Access Management: Provides connectivity through TCP/IP protocols and manages remote access for clients and administrators.
• Resource Allocation: Optimizes CPU, memory, and disk usage for high performance

Operating Systems (OS)

• An OS is software that manages hardware and software resources while providing a user interface.

Common Operating Systems

• Windows: User-friendly OS used primarily in personal computers and businesses, supporting GUI-based applications, gaming, and enterprise tools.
• Linux: An open-source OS used in servers, embedded systems, and desktops, known for being highly customizable and widely used in cloud computing.
• Unix: A multi-user, multitasking OS used in servers, workstations, and enterprise environments, known for stability, security, and scalability.
• macOS: Apple’s operating system for Mac computers, known for stability, security, and seamless integration with Apple’s ecosystem.

OS Architectures

• The OS has a layered structure represented as an onion model.
• Hardware Layer: Physical components of a computer include CPU, RAM, storage, and I/O devices.
• Kernel Layer: The core of the OS directly interacts with hardware, controls hardware through drivers/firmware and manages disk, tasks, processes, memory, device communication, system resources, security, and access control.
  • The kernel is the first program loaded on startup (after the bootloader) and provides resources to other apps (CPU, Memory, I/O, storage, etc.).
• Shell Layer: Provides a user interface for communicating with the operating system.
  • The shell layer is a command line interpreter (CLI) (e.g., Bash, Powershell) or a GUI (e.g., Windows Explorer, macOS Finder).
  • It enables users to interact with the Kernel using CLI and uses shell commands for file management, data processing, and permissions.
• Application Layer: Consists of software applications and utilities that communicate with users through the Shell which then communicates with hardware through the Kernel.
  • The Application Layer provides GUI software for user-friendly interaction with the Shell.

GUI vs CLI (Graphical vs Command Line Interface)

• GUI: Utilizes Windows, Icons, Menus, and Pointers (WIMP).
  • GUI is easy to use and visually appealing, requiring little experience and provides a safety net with specialized software.
  • GUI is prone to crashes, slower for repetitive tasks, requires more system resources, and makes users unaware of OS operations.
• CLI: Uses text commands to interact with the OS.
  • CLI is efficient for automation, requires fewer resources, is faster, can be run remotely, and is robust (difficult to crash).
  • CLI is hard to learn, lacks graphical elements and a safety net and has inconsistent commands and task options.

Unix Operating System - History & Evolution

• 1969: Unix was created, primarily used for networking.
• 1983: System V (AT&T's Unix version) was released.
• 1980s: BSD (Berkeley Software Distribution) variant was developed and distributed by the Computer Systems Research Group (CSRG).
  • Single Unix Specification (SUS) is an OS standard specifying programming interfaces for the C language, a command-line shell, and user commands.
• 1990s: The GNU project + Linux Kernel (1992) were created.
• 2000s: Commercial Linux distributions, such as RHEL and Ubuntu (2004), were released.

Unix vs Unix-like Systems

• Unix: Original systems and their derivatives conforming to the Unix standard.
• Unix-like: Systems inspired by Unix, including Linux & BSD, that are not necessarily certified Unix systems.

Strengths of Unix

• Simplicity: Small, modular programs that do one task well.
• Portability: Written in C making it easy to compile on different hardware.
• Efficiency: Optimized resource management with multitasking and multi-user capabilities.
• Security: Access control via permissions and user roles.

Unix Shell and Scripting Theory

• Shell as CLI Interpreter: Processes user commands and sends them to the kernel (e.g., Bash, Zsh, C Shell, Korn Shell).
• Benefits of Using Scripts: Automation reduces manual effort for repetitive tasks, and task batching executes multiple commands in sequence.
• General Scripting Flow: Involves writing (creating the script file), setting permissions (using chmod +x scriptname to make it executable), and executing (running with ./scriptname).
• Types of Scripts:
  • Bash Scripts: Common for Linux/Unix automation.
  • PowerShell Scripts: Windows-based scripting for system administration.
  • Python Scripts: Used for complex automation with programming capabilities.

Basic Understanding (Commands)

• Commands interact with the OS to perform actions such as file manipulation and process control
  • ls: List directory contents.
  • cd: Change directory.
  • pwd: Show current directory.
  • chmod: Change file permissions.
  • echo: Print text or variables.
  • cat: Display file contents.
  • grep: Search within files.
  • ps: Show running processes.
  • kill: Terminate a process.

Unix Filesystem

• Filesystem: A method used by an OS to store, retrieve, and organize files on storage devices, defining how data is stored and accessed.
• Logical Filesystem: Deals with user operations, such as opening, closing, reading, and writing files, and manages directories, file attributes, and permissions.
• Physical Filesystem: How items are physically stored and represented on storage.
  • Handles low-level storage operations, including how data is stored on disk sectors, and manages disk blocks, partitions, and disk scheduling.

Unix-Style Hierarchical Structure

• Unix follows a tree-like directory structure starting from the root (/).
• Root (/): The top-level directory.

Directories and Subdirectories

• /bin: Essential binary executables.
• /etc: Configuration files.
• /home: User home directories.
• /var: Variable data such as logs.
• /tmp: Temporary files.
• /dev: Device files.
• /mnt: Mounted file systems.
• /usr: User utilities and applications.

Storage Management

• Hard Drive Structure
  • Physical:
    • Tracks: Concentric rings on the platter.
    • Head: Reads data from a platter.
    • Cylinder: A collection of horizontal tracks on platters.
    • Spindle: Central rod that the disks or platters revolve around.
    • Sector: Smallest physical unit of HD Track (mostly 512 bytes).
    • Formatting: Prepares a disk for use by setting up a file system and creating partitions, enabling the OS to read and write data efficiently by organizing and marking the disk surface into tracks, sectors, and cylinders.
  • Logical:
    • Partitions: Divisions of the disk into independent sections that can hold filesystems. Each disk function acts as an independent storage unit.
    • Filesystem: Defines how data is stored, named, and retrieved (e.g., NTFS, ext4).
    • Sector: The smallest addressable storage unit on a disk, typically 512 bytes/4KB.
    • Cluster: A collection of blocks, treated as a single unit by the OS.
    • Block: The smallest fixed size unit of storage (group of consecutive sectors), used by filesystems as an allocation unit to read, write, and manage data.

Formatting - Preparing a Disk for Use

• Low-level formatting organizes the disk into tracks and sectors.
• Partitioning divides a disk into logical sections.
• High-level formatting creates a filesystem structure (e.g., ext4, NTFS, FAT32).

File Allocation Methods

• Contiguous Allocation: Files are stored in consecutive blocks, resulting in fast access but can lead to fragmentation over time.
  • Advantages: Easy implementation, faster read/write access time, minimal management needed.
  • Disadvantages: Can lead to fragmented space, predefined maximum file size, difficult to resize files.
• Linked Allocation: Each file is a linked list of blocks, with each block containing a pointer to the next block in the file.
  • Advantages: Efficient use of disk space without fragmentation and files can grow as needed by adding blocks, and files can be stored anywhere on the disk.
  • Disadvantages: Slower access due to pointer chasing, requires each block to store a pointer, increasing overhead, and is more complex to manage compared to contiguous allocation.
• Indexed Allocation: Each file has an index block containing pointers to its data blocks, enabling efficient access without following a pointer chain.
  • Advantages: Efficient use of disk space without fragmentation, direct access to data/file blocks, and files can be stored non-contiguously.
  • Disadvantages: Requires additional space for the index block, which has a limited size restricting the number of blocks a file can point to, and is more complex to manage compared to contiguous allocation.

Inodes (Unix File Management)

• Definition: A data structure used by Unix filesystems to store metadata about files.
• What it stores: File type, file size, permissions, owner and group information, timestamps, number of links, and disk block addresses where file data is stored.
• How the OS Uses Inodes: The OS looks up a file's inode to retrieve metadata when the file is accessed.
  • Each file has a unique inode number.
  • Inodes do not store the filename. Filenames are stored in directory entries that map to inodes.