Linux Kernel Architecture

Chapter 8: The Virtual Filesystem

Chapter 8: The Virtual Filesystem............................................

C-Standard Library (Libc)

Application Application Application

System-

calls

Userspace

Virtual Filesystem (VFS) Kernelspace

Ext2/3 Reiserfs XFS

Figure 8-1: VFS layer for filesystem abstraction.

8.1 Filesystem Types

Filesystems may be grouped into three general classes:

1. Disk-based filesystemsare the classic way of storing files on nonvolatile media to retain
   their contents between sessions. In fact, most filesystems have evolved from this category.
   Some well-known examples are Ext2/3, Reiserfs, FAT, and iso9660. All make use of block-
   oriented media and must therefore answer the question, how to store file contents and struc-
   ture information on the directory hierarchies. Of no interest to us here is the way in which
   communication takes place with the underlying block device — the corresponding device
   drivers in the kernel provide a uniform interface for this purpose. From the filesystem point
   of view, the underlying devices are nothing more than a list of storage blocks for which an
   appropriate organization scheme must be adopted.


2. Virtual filesystemsare generated in the kernel itself and are a simple way of enabling
   userspace programs to communicate with users. Theprocfilesystem is the best example of
   this class. It requires no storage space on any kind of hardware device; instead, the kernel
   creates a hierarchical file structure whose entries contain information on a particular part
   of the system. The file/proc/version, for example, has a nominal length of 0 bytes when
   viewed with thelscommand.
   wolfgang@meitner>ls -l /proc/version
   -r--r--r-- 1 root root 0 May 27 00:36 /proc/version
   However, if the file contents are output withcat, the kernel generates a list of infor-
   mation on the system processor; this list is extracted from the data structures in kernel
   memory.
   wolfgang@meitner>cat /proc/version
   Linux version 2.6.24 (wolfgang@schroedinger) (gcc version 4.2.1 (SUSE Linux))
   #1 Tue Jan 29 03:58:03 GMT 2008


3. Network filesystemsare a Halfway House between disk-based and virtual filesystems. They
   permit access to data on a computer attached to the local computer via a network. In this
   case, the data are, in fact, stored on a hardware device on a different system. This means
   that the kernel need not be concerned with the details of file access, data organization, and
   hardware communication — this is taken careof by the kernel of the remote computer. All
   operations on files in this filesystem are carried out over a network connection. When a pro-
   cess writes data to a file, the data are sent to the remote computer using a specific protocol