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Chapter 2.4 - Virtual File System (VFS)

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand the role of the VFS
  • Explain how VFS unifies different file systems
  • Understand inodes and data structures
  • Identify different types of file systems

What is the VFS?​

The VFS (Virtual File System) is an abstraction layer in the Linux Kernel that provides a unified interface for accessing different types of file systems, whether they are local (ext4, xfs) or network (NFS, CIFS).

Roles of the VFS​

1. Unified Interface​

The VFS provides a single interface for:

  • Opening, reading, writing, closing files
  • Creating, deleting, renaming files
  • Managing directories
  • Managing permissions

2. Abstraction​

Applications do not need to know:

  • The type of file system
  • The location (local or network)
  • Implementation details

3. Performance​

The VFS maintains caches:

  • Inode cache: File metadata
  • Dentry cache: File names and paths
  • Buffer cache: File data

Inodes​

An inode (index node) is a data structure that stores a file's metadata:

Stored information:

  • File type (file, directory, link, etc.)
  • Permissions (owner, group, others)
  • File size
  • Timestamps (creation, modification, access)
  • Pointers to data blocks
  • Number of links

Types of File Systems​

Local File Systems​

ext4 (Fourth Extended File System):

  • Standard on most distributions
  • Journaled (crash-resistant)
  • Supports files up to 16TB

xfs:

  • High performance for large files
  • Used in enterprise
  • Good for databases

btrfs:

  • Advanced features (snapshots, compression)
  • Copy-on-write
  • Actively developed

Network File Systems​

NFS (Network File System):

  • File sharing over network
  • Unix/Linux standard

CIFS/SMB:

  • Windows compatible
  • Cross-platform file sharing

Special File Systems​

tmpfs:

  • In memory (RAM)
  • Fast but temporary
  • /tmp, /dev/shm

procfs:

  • Interface to the kernel
  • /proc

sysfs:

  • System information
  • /sys

Directory Structure​

The VFS maintains a unified hierarchy:

/
├── bin/          # Essential binaries
├── etc/          # Configuration
├── home/         # Users
├── proc/         # Kernel interface (procfs)
├── sys/          # System interface (sysfs)
├── tmp/          # Temporary (tmpfs)
├── var/          # Variable data
└── ...

Useful Commands​

# View mounted file systems
mount
df -h

# View inodes
ls -li
stat file

# View inode usage
df -i

# View file system types
mount | grep -E '^/dev'

Summary​

In this chapter, you learned:

VFS: Unified interface for all file systems
Inodes: File metadata
Caches: Inode, Dentry, Buffer for performance
FS Types: Local (ext4, xfs), network (NFS), special (tmpfs, procfs)
Hierarchy: Unified structure with / as root

Next Steps​

Chapter 2.5: Process Management
Chapter 2.6: Init Process and systemd

Chapter created on: December 2024