File systems

The piece of the kernel that deals with file systems is the part that determines disk organization, for instance, what actual disk blocks on what device you need to access in order to perform a read, or directory lookup, or file creation. However, there are many different ways of organizing disks. In an attempt to make some of the facilities in the kernel share standard interfaces, there are some virtual layers , in file handling in particular, which insulate the basic kernel functions from the details about file system organization and file types.

In SunOS 4.x and Solaris 2, a VFS (Virtual File System) layer handles the basic details of mounting and unmounting file systems without worrying about their underlying structure. Device- or file-system -specific code is invoked indirectly to perform the needed operations for mounting, unmounting, getting statistics, or flushing data out to the device.

UNIX File Systems (UFS) are the most common systems, with NFS (Network File System) a close runner-up. A new entry in the list is HSFS, which handles a format compatible with DOS CD-ROM. Other new types include procfs to define and handle /proc , which in Solaris 2 is a pseudo-file system without any real files, just processes. All of these file system types have a common set of operations that must be performed, and specific code for each different type handles the details of how these operations are done. A Virtual File System structure contains a pointer to an array of functions that "do the job." Nobody cares, at the VFS layer, what kind of file system is underneath as long as the appropriate functions have been provided.