ch11

Chapter 10&11: File System Operating Systems

• Prepared by Silberschatz, Modified by Dr. Yuntong Zhang

11.1 Overview

11.2 File Attributes

• Name: The name of the file.

• Identifier: A unique tag (number) that identifies the file within the file system.

• Type: Required for systems that support different file types.

• Location: A pointer that indicates the file's location on the device.

• Size: The current size of the file.

• Protection: Controls the permissions for reading, writing, and executing the file.

• Time, Date, and User Identification: Information utilized for protection, security, and usage monitoring.

Typical File Control Block (FCB)

• Contains information about files.

11.3 File Types

• Executable: Typically has extensions like .exe, .com, .bin, indicating ready-to-run machine or binary language programs.

• Object Files: Extensions like .obj, .o, which are compiled machine language but not linked.

• Source Code: Extensions may include .c, .cc, .java, which represent source code in various programming languages.

• Batch Files: Extensions such as .bat, .sh which contain command scripts for command interpreters.

• Text Files: .txt, .doc represent textual data and documents.

• Libraries: .lib, .a, .so, .dll contain libraries of routines for programmers.

• Printable/Viewable Files: Formats like .ps, .pdf, .jpg are used for printable or viewable ASCII/binary files.

• Archives: Extensions such as .arc, .zip, .tar are used for storing related files together, often compressed for archiving.

• Multimedia: Extensions like .mpeg, .mov, .rm are binary files that contain audio or audiovisual information.

11.4 File Operations

• Files are considered an abstract data type with the following operations:

  • Create: Initiate a new file.

  • Write: Add data to a file.

  • Read: Retrieve data from a file.

  • Reposition: Move to a specific point within the file.

  • Delete: Remove the file from the system.

  • Truncate: Shorten the file.

  • Open(Fi): Search the directory structure for the entry Fi and load it into memory.

  • Close(Fi): Save changes from memory back to the disk.

11.5 Tree-Structured Directories

• Directory structures can have:

  • Absolute path: Full path from the root to the file.

  • Relative path: Starts from the current directory.

• File Management Commands:

  • Delete a file: Command syntax rm <file-name>.

  • Create a new folder: Command syntax mkdir <folder-name>.

  • Deleting directories: Deleting a directory will remove the entire subtree rooted by it.

11.6 Modern OS File-System Implementation

• On-disk structure:

  • Boot Control Block: Contains boot information.

  • Partition Control Block: Details of disk partitions.

  • Directory Structure: Information on directories.

  • File System Types: e.g., FAT, FAT32, NTFS.

• In-Memory File System Structures:

  • In-memory partition table: Information about mounted partitions.

  • In-memory directory structure: Holds recently accessed directory information.

  • System-wide open-file table: Keeps copies of FCB for all open files.

  • Per-process open-file table: Points to entries in the system-wide open-file table for each process.

11.7 Disk Block Allocation Methods

• A disk block: A fixed sequence of bytes on a disk, accessed as a single unit.

• Allocation methods:

  • Contiguous allocation: Files stored in adjacent blocks.

  • Linked allocation: Each file consists of scattered blocks, linked as a list.

  • Indexed allocation: Pointers to all file blocks are collected in a single index block.

11.8 Contiguous Allocation

• Files occupy a set of contiguous blocks:

  • Simplicity: Requires only the starting location and length.

  • Minimal seeks: Neighboring blocks reduce seek time.

  • Efficient for access types: Good for both sequential and direct access.

  • Drawbacks:

    • Inefficient space usage due to external fragmentation.

    • Files can't grow if adjacent blocks are occupied.

11.9 Example of Contiguous Allocation

• Example structure shows how contiguous allocation might organize files in a directory.

11.10 Linked Allocation

• Each file represented as a linked list of disk blocks scattered across the disk:

  • Directory holds pointers to the first and last blocks.

  • Advantages:

    • Easy to expand files.

  • Drawbacks:

    • Slower access speeds.

    • Decreased disk reliability; damage to a block can result in file loss.

    • Utilization of disk space is not optimal due to the need for pointers.

11.11 Indexed Allocation

• Pointers organized in an index block (an array of disk-block addresses):

  • Each file has its own index block.

  • Similar to paging in memory allocation schemes.

  • Essential for supporting random access but adds overhead with index blocks.

11.12 Free-Space Management

• Tracking free disk space using a Bitmap:

  • Each bit represents one disk block, indicating if it is free (0) or occupied (1).