On-Disk Data Structures in File Systems: Managing File Storage and Retrieval

Explore the on-disk data structures used by file systems to manage files. This guide explains key structures like the boot control block (BCB), volume control block (VCB), and inode table (in Unix-like systems) or Master File Table (MFT) (in NTFS), illustrating how they organize and track file information.

On-Disk Data Structures in File Systems

Understanding On-Disk Data Structures

File systems manage how files are stored and accessed on a disk. They use various on-disk data structures to organize and track file information. The specific structures vary between operating systems (like Unix, NTFS, etc.), but they all share the fundamental goal of efficiently storing and retrieving file data. These structures are essential for the operation of the file system.

Key On-Disk Data Structures

1. Boot Control Block

The boot control block (BCB) contains information needed to boot the operating system from that volume. In Unix-like systems, it's often called the boot block; in NTFS, it's the partition boot sector.

2. Volume Control Block (VCB)

The VCB stores information about the volume (a disk or partition), such as:

  • Total number of blocks.
  • Block size.
  • Partition table.
  • Pointers to free blocks.
  • Pointers to free File Control Blocks (FCBs).

In Unix, this is often called the superblock; in NTFS, it's in the Master File Table (MFT).

3. Directory Structure

The directory structure, specific to each file system, stores filenames and pointers to the corresponding File Control Blocks (FCBs). In Unix-like systems, it uses inode numbers to link filenames to file data.

4. File Control Block (FCB)

The FCB contains metadata about a file:

  • File name.
  • File size.
  • File type.
  • Owner information.
  • Permissions.
  • Pointers to data blocks.

In Unix-like systems, this information is in the inode; in NTFS, it's in the MFT.

Diagram of a typical File Control Block

(The image would show a diagram of a typical File Control Block (FCB), illustrating its various fields and their purposes.)