15 File System Internals

1 File Systems

Storage Devices can be sliced up into partitions, which hold volumes, which in turn hold file systems. A typical storage device organization:

2 File-System Mounting

A file system must be mounted before it can be available to processes on the system:

1. given the name of the device and the mount point(the location within the file structure where the file system is to be attached).
2. verifies that the device contains a valid file system, by asking the device driver to read the device directory and verifying that the directory has the expected format.
3. mounted at the specified mount point.

5 Virtual File Systems

ISSUE: How does an operating system allow multiple types of file systems to be integrated into a directory structure? And how can users seamlessly move between file-system types as they navigate the file-system space?

SOLUTION: virtual file system (VFS, 虚拟文件系统) layer which services two important functions:

• separates file-system-generic operations from their implementation by defining a clean VFS interface.
• provides a mechanism for uniquely representing a file throughout a network.
  • the VFS is based on a file-representation structure, called a vnode, that contains a numerical designator for a network-wide unique file. (UNIX inode are unique within only a single file system)

The file-system implementation consists of three major layers:

1. file-system interface: based on the open(), read(), write() and close() calls and on file descriptors
2. virtual file system (VFS) layer
3. local/remote file system

The four main object types defined by the Linux VFS are:

• inode (index node): represents an individual file
• file object: represents an open file
• superblock: represents an entire file system. It is essentially file system metadata and defines the file system type, size, status, and information about other metadata structures (metadata of metadata).
• dentry(directory entry): represents an individual directory entry