VFS
| Filesystems |
|---|
| Virtual Filesystems |
| Disk Filesystems |
| CD/DVD Filesystems |
| Network Filesystems |
| Flash Filesystems |
Introduction
The VFS or Virtual File System is used to provide transparent access to storage devices from applications. There are many different models for virtual file systems. Ranging from the DOS style of having a letter correspond to each file system to having a node list type. The VFS sits between the higher level operations like user space and process management and the file system drivers. The most important idea of the VFS is the Mount Point. A mount point is a place in the file system "address space" (valid path) that represents another file system. This file system may be on a local device, in memory or stored on a networked device.
VFS Models
DOS/Windows
The VFS model used in DOS and Microsoft Windows assigns a letter from the alphabet to each accessible file system on the machine. This type of VFS is the most simple to implement but is restricted to 26 mounted file systems and can get more and more complex as features are added. When a file is requested the VFS checks what drive the file is on and then passes the request on to the relevant driver.
Mount Point List
A more complex model is that of a mount point list. This system maintains a list of mounted file systems and where they are mounted. When a file is requested the list is scanned to determine what file system the file is on. The rest of the path is then passed on to the file system driver to fetch the file. This design is a quite versatile one but suffers from speed problems when large amounts of mount points are used.
Node Graph
A VFS model that can be very efficient is the Node Graph. This model maintains a graph of file system nodes that can represent a file, folder, mount point or other type of file. A node graph can be faster to traverse than a list but suffers from complexity problems and, if a large amounts of nodes are needed, can take up large amounts of memory. Each node in a node graph has the name, permissions and inode stored within a structure along with pointers to file IO functions like Read, Write, Read Dir and Find Dir.
Compromise
These models represent the basics for a VFS to be designed on, they have their problems however. Scanning through a list of mount points and then passing on to the file system the remainder of the path is usable for a simple OS but requires large amounts of repeated code as each driver must be able to parse a path reliably. A node graph on the other hand, requires a node for each file and directory on the system to present in memory at the same time, otherwise features like mount points would have to be constantly refreshed. A compromise between these two systems would be to have a list of mounted file systems and use that to determine what mount point a file lies on and then use nodes that do not necessarily have to permanently reside in memory to store file information and methods.