NAME

cfs − composite virtual file system

SYNOPSIS

#include <sys/mount.h>
mount ("cfs", dir, flags, model);

DESCRIPTION

The Composite File System (CFS) operates like the Sun Loopback File System except for these distinctions:

1.  The entire directory tree reachable from the mount point, including any exportable file systems mounted within it, may be exported to the network and accessed by clients as a single unified file system. 

2.  The statfs system call (see statfs(2)) always returns the statistics of the file system containing the mounted directory, rather than those of any other file system mounted within the directory tree. 

3.  Files may be represented by "transparent" links to permit network clients access to server files beyond the scope of the exported directory tree. 

A "transparent" link is a similar to a direct link, since it is traversed by the server rather than the client.  The linked-to file need not reside in the same file system as the link, but the link will present the appearance of that file for all purposes, whether locally or remotely initiated. 

Several varieties of "transparent" links are recognized.  A simple "transparent" link is an ordinary symbolic link with an absolute link path introduced by a double-slash (//) sequence.  Whenever such a link is accessed, the linked-to object is returned in place of the link. 

A composite "transparent" link or "alias" consists of two files.  The "alias" file contains the name of the linked-to file and is placed in a special "alias" directory and named with the index node (inode) number of the "target" file.  Whenever the empty "target" file is accessed, the "alias" file is consulted and the linked-to file is substituted. 

dir is the mount point for the composite file system.  model is the pathname of the existing file directory.  flags is either 0 or M_RDONLY.  The flag forces all accesses within the composite file system to be read-only; without it, accesses are the same as for the underlying file system.  All other mount(2V) flags are preserved from the underlying file system. 

Designed primarily to support optical disk jukeboxes, the Composite File System (CFS) allows a file directory tree containing many mounted file systems to be exported to the network as a single file system, and files within this tree to be represented by transparent links to other files beyond the scope of the exported directory tree. 

This directory tree acts as a "model" of the jukebox file structure.  Each directory in the model represents one or more directories with identical attributes and relative path names on different jukebox volumes, and contains transparent links to files in each corresponding directory on every volume in the jukebox.  Files are initially created in the model and subsequently transferred to a volume in the jukebox by a background process (see cfst(8)) and replaced by a transparent link. 

The model thus represents an aggregate structure formed as a logical "union" of the independent file structures on all participating volumes in the jukebox.  Each volume may be envisioned as a vertical "slice" of this structure.  This mechanism allows the distribution of files on various jukebox volumes to be completely concealed from the user. 

The file tree presented to the user resides on magnetic disk and comprises only directories and links to files on jukebox volumes.  Files appearing in the same directory may actually be located on several different volumes in the jukebox. 

File systems mounted within the model directory tree are also exported with the model, and may contain transparent links as well, but they are not truly part of the model; files created within them will not be transferred to jukebox volumes.  This feature allows jukebox volumes to be mounted within the model for direct access without affecting the utility of the model. 

The file system statistics reported are those of the model file system only, so they can be reliably used to gauge the space remaining for file creation without risk of data loss. 

FILES

model/.= directory index map

model/.:/<index> per-directory tally

model/.@ operation audit log

model/.~ temporary audit log

model/.! creation interlock

SEE ALSO

statfs(2), cfsi(8), cfse(8), cfst(8), cmount(8)

  —  30 September 1992