OS Notes

Linux File System - ext4 and others

Linux file systems — ext4 architecture, inodes, journaling, directory hierarchy (FHS), other file systems (XFS, Btrfs, ZFS), mounting, and file system management commands.

Introduction

The file system is how the operating system organizes data on storage devices — transforming raw disk blocks into the familiar hierarchy of files and directories. Linux supports dozens of file systems, each designed for different use cases. Understanding file system internals helps you choose the right one for your workload, troubleshoot performance issues, and recover from data corruption.

The beauty of Linux is the VFS (Virtual File System) layer — it provides a uniform interface so that applications use the same system calls (open, read, write) regardless of whether the underlying storage is ext4, XFS, NFS, or even a virtual filesystem like /proc.

Linux Directory Hierarchy (FHS)

The Filesystem Hierarchy Standard defines where everything lives:

/ ├── bin/ Essential user commands (ls, cp, grep) ├── boot/ Kernel and bootloader files ├── dev/ Device files (sda, tty, null, random) ├── etc/ System configuration files ├── home/ User home directories (/home/alice) ├── lib/ Shared libraries for /bin and /sbin ├── mnt/ Temporary mount points ├── opt/ Optional/third-party software ├── proc/ Virtual filesystem — process/kernel info ├── root/ Root user's home directory ├── sbin/ System administration commands ├── sys/ Virtual filesystem — hardware info ├── tmp/ Temporary files (cleared on reboot) ├── usr/ User programs, libraries, documentation │ ├── bin/ Non-essential user commands │ ├── lib/ Libraries for /usr/bin │ └── share/ Architecture-independent data └── var/ Variable data (logs, databases, mail, spool) ├── log/ System and application logs └── www/ Web server content

ext4 — The Default Linux File System

ext4 (Fourth Extended Filesystem) is the most widely used Linux file system. It evolved from ext2 → ext3 → ext4, gaining journaling, larger volume support, and better performance with each generation.

Key Features

Maximum file size: 16 TB
Maximum volume size: 1 EB (exabyte)
Journaling: Crash recovery without full fsck
Extents: Efficient storage for large files (contiguous block ranges)
Delayed allocation: Better block placement decisions
Backward compatible: Can mount ext3/ext2 volumes

Inode Structure

Every file and directory is represented by an inode containing metadata:

│ ├── 1 single indirect pointer (	block of pointers → data)
│ ├── 1 double indirect pointer (	block → block of ptrs → data)
│ └── 1 triple indirect pointer (	block → block → block → data)

Important: The filename is NOT stored in the inode. It is stored in the parent directory's data blocks as a (name, inode_number) pair. This is why:

Hard links work (multiple names → same inode)
Renaming is instant (change directory entry, not file data)
ls -i shows inode numbers

Journaling

ext4 uses a journal to prevent file system corruption after crashes:

Without journaling

1. Start writing file metadata

2. CRASH mid-write

3. File system in inconsistent state

4. Full fsck required (can take hours on large volumes)

With journaling

1. Write intended changes to journal

2. Apply changes to actual file system

3. Mark journal entry as complete

4. CRASH at any point →

On recovery: replay complete journal entries, discard incomplete ones

Fast recovery in seconds

Journal modes:

journal: Both metadata AND data journaled (safest, slowest)
ordered (default): Only metadata journaled, but data written before metadata (good balance)
writeback: Only metadata journaled, data may be written in any order (fastest, riskier)

Other Linux File Systems

XFS

Developed by SGI for large files and high-throughput workloads:

Excellent for large files (video editing, databases)
Parallel I/O with allocation groups (each AG is independently managed)
Cannot be shrunk (only grown) — major limitation
Default on RHEL/CentOS since version 7

bash example

mkfs.xfs /dev/sdb1
mount /dev/sdb1 /data
xfs_growfs /data          # Expand to fill partition

Btrfs (B-tree File System)

Modern copy-on-write file system with advanced features:

Snapshots: Instant point-in-time copies (zero initial space)
Subvolumes: Independent file system namespaces within one partition
Built-in RAID: Software RAID 0, 1, 5, 6, 10
Checksums: Detects silent data corruption (bit rot)
Compression: Transparent compression (zstd, lzo, zlib)

bash example

mkfs.btrfs /dev/sdb1
# Create subvolume
btrfs subvolume create /mnt/data/@home
# Take snapshot
btrfs subvolume snapshot /mnt/data/@home /mnt/data/@home_backup
# Enable compression
mount -o compress=zstd /dev/sdb1 /mnt/data

ZFS

Originally from Sun Solaris, now available on Linux (via module):

Combined volume manager + file system
Strong data integrity (checksums on all data and metadata)
Copy-on-write (never overwrites data in place)
Built-in compression, deduplication, encryption
Snapshot and clone capabilities

bash example

# Create pool from multiple disks
zpool create mypool mirror /dev/sdb /dev/sdc
# Create dataset (like a filesystem)
zfs create mypool/data
# Snapshot
zfs snapshot mypool/data@today
# Enable compression
zfs set compression=lz4 mypool/data

tmpfs

RAM-based file system for temporary data:

bash example

mount -t tmpfs -o size=2G tmpfs /tmp
# Files in /tmp stored in RAM — extremely fast, lost on reboot

File System Operations

bash example

# Create file system
mkfs.ext4 /dev/sdb1
mkfs.xfs /dev/sdb1

# Mount
mount /dev/sdb1 /mnt/data
mount -t nfs server:/share /mnt/nfs     # Network mount

# Persistent mount (/etc/fstab)
/dev/sdb1  /data  ext4  defaults  0  2

# Check and repair
fsck /dev/sdb1          # Check (must be unmounted!)
e2fsck -f /dev/sdb1     # Force check ext4

# View file system info
df -hT                  # Disk usage with filesystem types
tune2fs -l /dev/sdb1    # ext4 superblock information
stat file.txt           # Inode details for a file

File System Comparison

Feature	ext4	XFS	Btrfs	ZFS
Max file size	16 TB	8 EB	16 EB	16 EB
Journaling	Yes	Yes	CoW	CoW
Snapshots	No	No	Yes	Yes
Checksums	Metadata only	No	Yes	Yes
Compression	No	No	Yes	Yes
Shrink volume	Yes	No	Yes	No
Maturity	Very mature	Mature	Maturing	Very mature
Best for	General purpose	Large files	Flexible storage	Data integrity

Key Takeaways

Linux VFS provides a uniform interface over diverse file systems
ext4 is the reliable default — journaling, extents, good performance for most workloads
XFS excels for large files and high-throughput scenarios
Btrfs and ZFS offer modern features (snapshots, checksums, compression) for advanced needs
Inodes store metadata; directory entries store name-to-inode mappings
Journaling prevents corruption after crashes — recovery takes seconds instead of hours
Choose file system based on workload: general purpose (ext4), large files (XFS), advanced features (Btrfs/ZFS)

Exam Focus

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OS Notes

Linux File System - ext4 and others

Linux file systems — ext4 architecture, inodes, journaling, directory hierarchy (FHS), other file systems (XFS, Btrfs, ZFS), mounting, and file system management commands.

Introduction

Linux Directory Hierarchy (FHS)

The Filesystem Hierarchy Standard defines where everything lives:

ext4 — The Default Linux File System

Key Features

Maximum file size: 16 TB
Maximum volume size: 1 EB (exabyte)
Journaling: Crash recovery without full fsck
Extents: Efficient storage for large files (contiguous block ranges)
Delayed allocation: Better block placement decisions
Backward compatible: Can mount ext3/ext2 volumes

Inode Structure

Every file and directory is represented by an inode containing metadata:

│ ├── 1 single indirect pointer (	block of pointers → data)
│ ├── 1 double indirect pointer (	block → block of ptrs → data)
│ └── 1 triple indirect pointer (	block → block → block → data)

Important: The filename is NOT stored in the inode. It is stored in the parent directory's data blocks as a (name, inode_number) pair. This is why:

Hard links work (multiple names → same inode)
Renaming is instant (change directory entry, not file data)
ls -i shows inode numbers

Journaling

ext4 uses a journal to prevent file system corruption after crashes:

Without journaling

1. Start writing file metadata

2. CRASH mid-write

3. File system in inconsistent state

4. Full fsck required (can take hours on large volumes)

With journaling

1. Write intended changes to journal

2. Apply changes to actual file system

3. Mark journal entry as complete

4. CRASH at any point →

On recovery: replay complete journal entries, discard incomplete ones

Fast recovery in seconds

Journal modes:

journal: Both metadata AND data journaled (safest, slowest)
ordered (default): Only metadata journaled, but data written before metadata (good balance)
writeback: Only metadata journaled, data may be written in any order (fastest, riskier)

Other Linux File Systems

XFS

Developed by SGI for large files and high-throughput workloads:

Excellent for large files (video editing, databases)
Parallel I/O with allocation groups (each AG is independently managed)
Cannot be shrunk (only grown) — major limitation
Default on RHEL/CentOS since version 7

bash example

mkfs.xfs /dev/sdb1
mount /dev/sdb1 /data
xfs_growfs /data          # Expand to fill partition

Btrfs (B-tree File System)

Modern copy-on-write file system with advanced features:

Snapshots: Instant point-in-time copies (zero initial space)
Subvolumes: Independent file system namespaces within one partition
Built-in RAID: Software RAID 0, 1, 5, 6, 10
Checksums: Detects silent data corruption (bit rot)
Compression: Transparent compression (zstd, lzo, zlib)

bash example

mkfs.btrfs /dev/sdb1
# Create subvolume
btrfs subvolume create /mnt/data/@home
# Take snapshot
btrfs subvolume snapshot /mnt/data/@home /mnt/data/@home_backup
# Enable compression
mount -o compress=zstd /dev/sdb1 /mnt/data

ZFS

Originally from Sun Solaris, now available on Linux (via module):

Combined volume manager + file system
Strong data integrity (checksums on all data and metadata)
Copy-on-write (never overwrites data in place)
Built-in compression, deduplication, encryption
Snapshot and clone capabilities

bash example

# Create pool from multiple disks
zpool create mypool mirror /dev/sdb /dev/sdc
# Create dataset (like a filesystem)
zfs create mypool/data
# Snapshot
zfs snapshot mypool/data@today
# Enable compression
zfs set compression=lz4 mypool/data

tmpfs

RAM-based file system for temporary data:

bash example

mount -t tmpfs -o size=2G tmpfs /tmp
# Files in /tmp stored in RAM — extremely fast, lost on reboot

File System Operations

bash example

# Create file system
mkfs.ext4 /dev/sdb1
mkfs.xfs /dev/sdb1

# Mount
mount /dev/sdb1 /mnt/data
mount -t nfs server:/share /mnt/nfs     # Network mount

# Persistent mount (/etc/fstab)
/dev/sdb1  /data  ext4  defaults  0  2

# Check and repair
fsck /dev/sdb1          # Check (must be unmounted!)
e2fsck -f /dev/sdb1     # Force check ext4

# View file system info
df -hT                  # Disk usage with filesystem types
tune2fs -l /dev/sdb1    # ext4 superblock information
stat file.txt           # Inode details for a file

File System Comparison

Feature	ext4	XFS	Btrfs	ZFS
Max file size	16 TB	8 EB	16 EB	16 EB
Journaling	Yes	Yes	CoW	CoW
Snapshots	No	No	Yes	Yes
Checksums	Metadata only	No	Yes	Yes
Compression	No	No	Yes	Yes
Shrink volume	Yes	No	Yes	No
Maturity	Very mature	Mature	Maturing	Very mature
Best for	General purpose	Large files	Flexible storage	Data integrity

Key Takeaways

Linux VFS provides a uniform interface over diverse file systems
ext4 is the reliable default — journaling, extents, good performance for most workloads
XFS excels for large files and high-throughput scenarios
Btrfs and ZFS offer modern features (snapshots, checksums, compression) for advanced needs
Inodes store metadata; directory entries store name-to-inode mappings
Journaling prevents corruption after crashes — recovery takes seconds instead of hours
Choose file system based on workload: general purpose (ext4), large files (XFS), advanced features (Btrfs/ZFS)

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Linux File System - ext4 and others.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Operating Systems topic.

Search Terms

operating-systems, operating systems, operating, systems, linux, and, unix, file

Continue learning this concept

PreviousBasic Linux Commands - Command Line Usage NextLinux Introduction - OS Overview