Virtual File Systems (VFS)

Unix Filesystems

Historically, Unix has provided four basic filesystem-related abstractions: files, directory entries, inodes, and mount points.

A filesystem is a hierarchical storage of data adhering to a specific structure. A filesystem type determines how information is organized on a device to represent a tree of directories and files. Filesystems contain files, directories, and associated control information. Typical operations performed on filesystems are creation, deletion, and mounting.

In Unix, filesystems are mounted at a specific mount point in a global hierarchy known as a namespace. This enables all mounted filesystems to appear as entries in a single tree. Windows break the file namespace up into drive letters, such as C:. This breaks the namespace and leaks hardware details into the filesystem abstraction.

A file is an ordered string of bytes. Each file has a human-readable name for identification by both the system and the user. Typical file operations are read, write, create, and delete.

Files are organized in directories. A directory is analogous to a folder and usually contains related files. Directories can also contain other directories, called subdirectories. In this fashion, directories may be nested to form paths. Each component of a path is called a directory entry, or dentries. In Unix, directories are actually normal files that simply list the files contained therein. Because a directory is a file to the VFS, the same operations performed on files can be performed on directories.

Unix systems separate the concept of a file from any associated information about it, such as access permissions, size, owner, creation time, and so on. This information is sometimes called file metadata (that is, data about the file’s data) and is stored in a separate data structure from the file, called the inode.

All this information is tied together with the filesystem’s own control information, which is stored in the superblock (filesystem metadata). The superblock is a data structure containing information about the filesystem as whole (individual files and the filesystem as a whole).

Traditionally, Unix filesystems implement these notions as part of their physical disk layout. Non-Unix filesystems, such as FAT, still work in Linux, but their filesystem code must provide the apperance of these concepts with processing done on-the-fly.

VFS Objects and Their Data Structures

The VFS is object-oriented.

The four primary object types of the VFS are:

  • The superblock object, which represents a specific mounted filesystem.

  • The inode object, which represents a specific file.

  • The denetry object, which represents a directoy entry, which is a single component of a path.

  • The file object, which represents an open file as associate with a process.

An operations object is contained within each of these primary objects. These objects describe the methods that the kernel invokes against the primary objects. The operations objects are implemented as a structure of pointers to functions that operate on the parent object.

Each registered filesystem is represented by a file_system_type structure. This object describes the filesystem and its capabilities.

Each mount point is represented by the vfsmount structure. This structure contains information about the mount point, such as its location and mount flags.

Two per-process structures describe the filesystem and files associated with a process. They are, respectively, the fs_struct structure and the file structure.

procfs vs sysfs

procfs (Process File System)

procfs is a virtual filesystem that exposes process and kernel runtime information.

  • Mount point: /proc
  • Focus: Processes + system state
  • Created dynamically by the kernel

Examples

  • /proc/cpuinfo
  • /proc/meminfo
  • /proc/<pid>/

Key Idea

Used to inspect running processes and overall system status.

sysfs (System File System)

sysfs is a virtual filesystem that exposes the Linux device and driver model.

  • Mount point: /sys
  • Focus: Devices + drivers
  • Introduced in Linux 2.6

Examples

  • /sys/class/
  • /sys/devices/
  • /sys/bus/

Key Idea

Used to represent hardware and kernel objects in a structured way.

Quick Difference

  • procfs → Process and kernel state
  • sysfs → Devices and driver model

What are the main files systems supported by the Linux kernel?

  • ext4 file system: provides journaling and support for large file sizes and partitions.

  • XFS file system: high-performance file system designed for large-scale storage systems.

  • Btrfs file system: provides features such as snapshotting and subvolumes.

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