5.1. Introduction to Filesystems
On a UNIX system, all files exist in a tree of directories under the root / directory. Drive letters used by other operating systems (like Windows) to identify different hard disks or network drives do not exist. Instead, different hard disks, CD-ROMs, floppy disks, and network drives are attached to the directory tree at different places, called mount points. For example, /home may be a mount point for a different hard disk on your system, and /usr/local may be the mount point for files that are shared from another server. The root directory is also a mount point, almost always for a partition on a hard disk in your machine. The set of files that is actually mounted at a mount point is called a filesystem.
All operating systems divide each hard disk up into partitions, each of which can be a different size. Each filesystem is normally stored on one partition of one disk, so it is possible to have multiple filesystems of different types on the same hard disk—one for Linux and one for Windows, for example. If you have multiple hard disks in your system, you will normally need to mount at least one filesystem from each in order to make use of them.
UNIX systems support many different kinds of filesystems—some for files stored on local hard disks and some for files on networked file servers. On Linux, the filesystems on your hard disks will probably be in ext2 or ext3 format. Many other local filesystem types exist, such as iso-9660 for CD-ROMs, vfat for Windows partitions, and xfs and reiserfs for high performance file access. Every local filesystem type uses a different format for storing data on disk, so if a partition has been formatted as a filesystem of a particular type, then it must be mounted as that type.
There are also filesystem types for different methods of accessing file servers across a network. If the file server is running UNIX, then an nfs filesystem is usually mounted to access its files. However, if it is running Windows, an smbfs filesystem must be used instead. These different filesystem types correspond to different network protocols for accessing files on another system.
Other special filesystem types contain files that do not actually exist on any disk or file server. For example, a proc filesystem contains files that contain information about currently running processes. Different UNIX variants have different types of special filesystems, most of which are automatically mounted by the operating system and do not need to be configured.
No explanation of filesystems can be complete without also covering virtual memory. Often a UNIX system will be running processes that take up more memory than is actually installed. This is made possible by the operating system automatically moving some of those processes out of real memory and into virtual memory, which is stored in a file or a local hard disk. Because filesystems and virtual memory are both stored on disk and can be mounted and unmounted, the Disk and Network Filesystems Webmin module also manages with virtual memory.
Depending on your operating system, the files /etc/fstab or /etc/vfstab contain a list of filesystems that are known to your system and mounted at boot time. It is also possible for a filesystem to be temporarily mounted using the mount command without being stored in the fstab file. Webmin directly modifies this file to manage filesystems that are mounted at boot time, and calls the mount and unmount commands to immediately activate and deactivate filesystems.