OS File Concepts and Access Methods

OS File Concepts

• File attributes include name, identifier, type, location, size, protection, timestamps, and user identification.
• File operations involve creating, opening, writing, reading, repositioning within a file, deleting, and truncating.
• Contiguous allocation stores a file as a contiguous data block on a storage device being efficient but can lead to fragmentation.
• Linked allocation stores a file as a linked list of data blocks on a storage device. Each block contains a pointer to the next, which is flexible for any file size but slower on access.
• Indexed allocation uses an index to store addresses of the data blocks making up a file, speeding access, and reducing fragmentation.
• Combined allocation uses contiguous and linked allocation elements. The file stores data in a contiguous block until a size is reached, then stores it using linked allocation.

Access Methods

• Sequential access is when data is accessed sequentially; reading data from the beginning of a file or storage device, commonly used for tapes.
• Direct access enables reading data from any point in a file or storage device, often used for hard disk drives.
• Random access allows direct and quick access to any location in a storage device, as implemented in RAM and cache, where data can be accessed in any order.
• Indexed access builds on sequential access using an index to locate data within a file or storage device, containing pointers to the data locations.

Directory Structure

• In a single-level directory structure all files are stored in a single directory, which is simple but becomes unmanageable as file numbers grow.
• The two-level directory structure employs a root directory containing multiple user directories with individual file sets, which offers better organization but still presents management challenges with scaling.
• Hierarchical directory structure uses a tree-like structure to organize files, where each directory can hold files or subdirectories, providing logical organization.
• Indexed directory structure uses an index to organize files, enabling quick access to specific files without a full scan of the directory.
• Virtual file system is an interface for accessing different types of files and storage devices allowing the OS to manage all types of files with a single structure.

OS File System

• A file system is a method for organizing and storing files on a computer’s storage devices, such as hard disks, SSDs, and USB drives.
• A file system gives structure for the storage and retrieval of files, directories, and other data on a storage device.
• File systems comprise a low-level format defining how a physical storage device is partitioned and organized into sectors, and a high-level format defining how files/directories are stored/accessed.
• In Linux, everything is a file.

File Systems

• FAT (File Allocation Table) is a simple file system used by older versions of Microsoft Windows and other systems, it organizes files into clusters using a file allocation table to track used clusters by each file.
• NTFS (New Technology File System) is the primary file system used by modern Windows versions. It includes file permissions, encryption, and compression.
• HFS+ (Hierarchical File System Plus) is the file system used by Mac OS X. Features include file permissions, journaling, and extended attributes.
• ext4 (Fourth Extended File System) is the default file system used by most Linux distributions. It features journaling, file permissions, and supports large file/partition sizes.
• APFS (Apple File System) is the default used by macOS High Sierra and later. It's optimized for SSDs and features snapshots, cloning, and encryption.

File System Structure

• Hierarchical: Widely used by operating systems is a tree-like structure with a root directory where subdirectories branch.
• Flat File: Files are stored without subdirectories, less scalable and difficult to manage as file numbers grow.
• Network: Allows multiple computers to access and share files on a central server.
• Distributed: Files are stored across multiple servers in a network, accessible as if on a single server.
• Journaling: Prevents data loss by logging changes, ensuring quick recovery from crashes or power failures.

File System Operation

• Create: Allows a user or application to create a new file or directory.
• Delete: Removes a file or directory.
• Read: Allows a user/application to read a file.
• Write: Permits a user/application to write new data, replacing/appending to existing file content.
• Rename: Changes the name of a file/directory.
• Move: Relocates a file/directory to a new location.
• Copy: Creates a duplicate of a file/directory.
• List: Allows viewing of files/directories.
• Attribute Change: Modifies file/directory attributes, such as permissions/ownership.

The Linux File System

• In Linux, files and directories are represented with a tree-like structure with the topmost directory referred to as the root directory (/).
• Files can be accessed by specifying the path. Understanding the file system structure makes this easier.
• A path is the string of directories/sub-directories navigated to reach a given location. Paths can be specified using two methods: absolute or relative.
• Absolute Path: Complete path to a file starting from the root directory.
• Relative Path: Specifies a file location relative to the current working direction.

Linux File System Architecture

• Linux file system consists of 3 layers: logical, virtual, and physical.
• Logical File System acts as the interface between the user applications and the file system itself. It facilitates essential operations such as opening, reading, and closing files.
• Virtual File System is an essential layer that enables the concurrent operation of multiple instances of physical file systems.
• Physical File System is responsible for the tangible management of physical memory blocks on the disk by handling low-level storage/retrieving data, interacting with the hardware.

Linux Directory Structure