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SerenityOS features a flexible Virtual File System (VFS) architecture that supports multiple file system types, both traditional and special-purpose file systems.

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Overview

The kernel provides a unified VFS layer that allows different file system implementations to coexist and interact seamlessly. Applications access files through standard POSIX APIs regardless of the underlying file system type.

VFS Layer

Unified interface for all file systems

Multiple Types

Ext2, FAT, ISO9660, and more

Special FS

procfs, sysfs, devfs for system information

POSIX Compatible

Standard file operations and semantics

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Supported File Systems

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Ext2 File System

The primary file system used by SerenityOS.
Capabilities:
  • Block-based storage
  • Inode-based file representation
  • Directory hierarchies
  • Symbolic and hard links
  • File permissions (Unix-style)
  • Extended attributes
  • File timestamps (atime, mtime, ctime)
  • Large file support
Performance:
  • Block caching
  • Read-ahead optimization
  • Deferred writes
  • Buffer cache management
Reliability:
  • Journaling support (planned)
  • fsck utilities for repair
  • Consistent metadata updates
Why Ext2?
  • Well-documented specification
  • Proven track record
  • Simple and reliable
  • Good performance characteristics
  • Easy to implement correctly

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FAT File System

FAT16/FAT32 support for compatibility with other operating systems.
Supported Variants:
  • FAT12 (floppy disks)
  • FAT16 (small partitions)
  • FAT32 (large partitions)
Features:
  • Read and write support
  • Long filename support (VFAT/LFN)
  • Cross-platform compatibility
  • USB drive access
  • SD card support
Limitations:
  • No permissions or ownership
  • Limited metadata
  • Case-insensitive on some variants
  • 4GB file size limit (FAT32)
Use Cases:
  • USB flash drives
  • SD cards
  • Shared storage with other OSes
  • Boot partitions

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ISO 9660 (CD-ROM)

Read-only file system for CD/DVD media. Features:
  • ISO 9660 standard compliance
  • Rock Ridge extensions (Unix metadata)
  • Joliet extensions (long filenames)
  • El Torito bootable CD support
  • Multi-session support
Use Cases:
  • Live CD/DVD boots
  • Software distribution
  • Archived data
  • Installation media

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Plan9FS

9P protocol file system for distributed computing. Features:
  • Network file system protocol
  • Resource sharing
  • Simple protocol design
  • Virtual file system interface
Use Cases:
  • Remote file access
  • Distributed systems
  • Development and testing

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Special File Systems

SerenityOS provides several special-purpose file systems for system introspection and device access.

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ProcFS (/proc)

Process and system information file system.
Per-Process Information:
  • /proc/[pid]/cmdline - Command line arguments
  • /proc/[pid]/exe - Executable path
  • /proc/[pid]/cwd - Current working directory
  • /proc/[pid]/fd/ - Open file descriptors
  • /proc/[pid]/vm - Virtual memory layout
  • /proc/[pid]/unveil - Unveiled paths
  • /proc/[pid]/pledge - Pledged promises
  • /proc/[pid]/fds - File descriptor information
  • /proc/[pid]/stacks - Thread stack traces
  • /proc/[pid]/perf_events - Performance counters
  • /proc/[pid]/children - Child processes
System-Wide Information:
  • /proc/cpuinfo - CPU information
  • /proc/memstat - Memory statistics
  • /proc/uptime - System uptime
  • /proc/cmdline - Kernel command line
  • /proc/modules - Loaded kernel modules
  • /proc/net/ - Network statistics
  • /proc/sys/ - Kernel parameters
  • /proc/interrupts - Interrupt counters
  • /proc/pci - PCI devices
  • /proc/devices - Device list
Use Cases:
  • System monitoring tools (SystemMonitor)
  • Process management
  • Debugging and profiling
  • System information gathering

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SysFS (/sys)

Kernel and device tree information.
Device Information:
  • /sys/bus/ - Bus types and devices
  • /sys/class/ - Device classes
  • /sys/devices/ - Device hierarchy
  • /sys/firmware/ - Firmware information
  • /sys/kernel/ - Kernel parameters
  • /sys/power/ - Power management
Hardware Discovery:
  • Device enumeration
  • Driver binding information
  • Hardware capabilities
  • Device configuration
Use Cases:
  • Hardware discovery
  • Device management
  • Kernel parameter tuning
  • Power management control

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DevFS (/dev)

Device file system for hardware access.
Character Devices:
  • /dev/null - Null device
  • /dev/zero - Zero device
  • /dev/random - Random number generator
  • /dev/urandom - Non-blocking random
  • /dev/tty - Controlling terminal
  • /dev/pts/* - Pseudo-terminals
  • /dev/console - System console
  • /dev/kbd - Keyboard
  • /dev/mouse - Mouse
  • /dev/audio - Audio device
Block Devices:
  • /dev/hd* - Hard disks
  • /dev/sd* - SCSI/SATA disks
  • /dev/loop* - Loop devices
  • /dev/ramdisk* - RAM disks
Special Devices:
  • /dev/fb0 - Framebuffer
  • /dev/dsp - Audio DSP
  • /dev/mixer - Audio mixer
  • /dev/input/* - Input devices

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DevPtsFS (/dev/pts)

Pseudo-terminal slave devices. Features:
  • Dynamic PTY allocation
  • Per-session isolation
  • Terminal emulation support
  • Job control support

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TmpFS (/tmp)

Temporary file storage in RAM. Features:
  • Memory-backed storage
  • Fast access speeds
  • Automatic cleanup on reboot
  • Configurable size limits
  • No disk I/O overhead
Use Cases:
  • Temporary file storage
  • Cache files
  • Build artifacts
  • Socket files
  • Session data

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RAMFS

Simple RAM-based file system. Features:
  • Entirely in memory
  • No persistence
  • Fast operations
  • Dynamic sizing
Use Cases:
  • Initial ramdisk (initrd)
  • Testing and development
  • Volatile storage needs

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DevLoopFS

Loop device file system for mounting files as block devices. Features:
  • Mount regular files as disks
  • Support for disk images
  • Transparent access
  • Multiple concurrent loop devices

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FUSE (Filesystem in Userspace)

Framework for user-space file system implementations. Features:
  • User-space file system development
  • No kernel module required
  • Easier debugging
  • Sandboxed operation

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Virtual File System (VFS)

The VFS layer provides a unified interface for all file system types.

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VFS Components

Inodes

File and directory representation

Dentries

Directory entry cache

File Objects

Open file instances

Superblocks

File system metadata

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VFS Operations

Inode Operations:
  • File creation and deletion
  • Link and unlink
  • Rename operations
  • Permission changes
  • Attribute queries
File Operations:
  • Open and close
  • Read and write
  • Seek operations
  • Memory mapping
  • I/O control (ioctl)
Directory Operations:
  • Directory traversal
  • Entry lookup
  • Directory creation
  • Directory removal

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File System Features

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Permissions & Security

Unix Permissions:
  • Owner, group, other permissions
  • Read, write, execute bits
  • Special bits (setuid, setgid, sticky)
  • Numeric (octal) and symbolic modes
Access Control:
  • User and group ownership
  • Permission checking
  • Root privilege handling
  • File mode enforcement

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File Attributes

Standard Attributes:
  • File size
  • Timestamps (access, modification, change)
  • Owner and group IDs
  • Permission bits
  • File type
  • Link count
Extended Attributes:
  • Arbitrary key-value metadata
  • Security labels
  • User-defined attributes

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File Types

  • Regular files - Normal data files
  • Directories - File containers
  • Symbolic links - Name aliases
  • Hard links - Additional names
  • Character devices - Character I/O
  • Block devices - Block I/O
  • FIFOs (named pipes) - IPC mechanism
  • Sockets - Network/IPC endpoints

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File System Operations

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Mounting

# Mount a file system
mount /dev/hda1 /mnt/data

# Mount with options
mount -o ro /dev/cdrom /mnt/cd

# Mount by type
mount -t ext2 /dev/sda2 /home

# View mounted file systems
mount

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Unmounting

# Unmount a file system
umount /mnt/data

# Force unmount
umount -f /mnt/data

# Lazy unmount
umount -l /mnt/data

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File System Check

# Check Ext2 file system
e2fsck /dev/hda1

# Check and repair
e2fsck -p /dev/hda1

# Verbose check
e2fsck -v /dev/hda1

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Caching & Performance

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Buffer Cache

The kernel maintains a buffer cache for improved I/O performance: Features:
  • Block-level caching
  • Write-back caching
  • Read-ahead optimization
  • LRU eviction policy
  • Dirty buffer tracking
  • Periodic sync to disk

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Page Cache

Memory-mapped file caching:
  • File content caching
  • Shared page mappings
  • Copy-on-write pages
  • Memory pressure handling
  • Demand paging

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Inode Cache

In-memory inode storage:
  • Active inode caching
  • Quick metadata access
  • Reduced disk reads
  • LRU-based eviction

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File System Utilities

Tools for managing file systems: Creation:
  • mkfs.ext2 - Create Ext2 file system
  • mke2fs - Extended Ext2 creation
Checking:
  • fsck - Generic file system check
  • e2fsck - Ext2-specific check
Information:
  • df - Disk space usage
  • du - Directory space usage
  • stat - File status
  • file - File type detection
Management:
  • mount / umount - Mount operations
  • sync - Flush caches to disk
  • blockdev - Block device control

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POSIX Compatibility

SerenityOS provides POSIX-compliant file operations: System Calls:
  • open(), close()
  • read(), write()
  • lseek()
  • stat(), fstat(), lstat()
  • mkdir(), rmdir()
  • link(), unlink()
  • symlink(), readlink()
  • rename()
  • chmod(), chown()
  • truncate(), ftruncate()
  • mmap(), munmap()
File Descriptors:
  • Standard streams (stdin, stdout, stderr)
  • Descriptor inheritance
  • Close-on-exec flag
  • Non-blocking I/O
  • Asynchronous I/O (in development)

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File Watching

File system change notification: Features:
  • File modification watching
  • Directory content monitoring
  • Create/delete notifications
  • Attribute change events
  • Efficient event delivery
Use Cases:
  • File manager updates
  • Build system triggers
  • Automatic reloading
  • Synchronization tools
File system notifications are implemented efficiently in the kernel and delivered to userspace applications via a special file descriptor interface.

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Storage & Partitioning

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Partition Support

Support for various partition schemes:
  • MBR (Master Boot Record) - Traditional PC partitioning
  • GPT (GUID Partition Table) - Modern UEFI partitioning
  • Multiple partitions per disk
  • Extended partitions
  • Partition type detection

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Disk Management

Tools:
  • PartitionEditor - GUI partitioning tool
  • fdisk - Command-line partitioning
  • blockdev - Block device utilities
Features:
  • Partition creation and deletion
  • Partition resizing
  • File system formatting
  • Disk information display

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File Format Support

SerenityOS can read and write numerous file formats: Archives:
  • TAR (Tape Archive)
  • ZIP compression
  • GZIP compression
Images:
  • PNG, JPEG, GIF, BMP
  • QOI (Quite OK Image)
  • ICO (Windows Icon)
  • TGA (Targa)
Documents:
  • PDF viewing
  • Markdown rendering
  • Plain text
Media:
  • WAV, MP3, FLAC audio
  • Video formats (in development)
The file system architecture is designed to be extensible, allowing new file system types to be added easily through the VFS interface.

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