.\" Copyright (c) 1992 The Regents of the University of California
.\" This code is derived from software donated to Berkeley by
.\" John Heidemann of the UCLA Ficus project.
.\" %sccs.include.redist.roff%
.\" @(#)mount_null.8 5.2 (Berkeley) %G%
.Nd demonstrate the use of a null file system layer
null layer, duplicating a sub-tree of the file system
name space under another part of the global file system namespace.
similar to the loopback file system (see
the loopback file system in two respects: it is implemented using
a stackable layers techniques, and it's
all lower-layer vnodes, not just over directory vnodes.
The null layer has two purposes. First, it serves as a demonstration
of layering by proving a layer which does nothing. (It actually
does everything the loopback file system does, which is slightly
more than nothing.) Second, the null layer can serve as a prototype
layer. Since it provides all necessary layer framework,
new file system layers can be created very easily be starting
The remainder of this man page examines the null layer as a basis
for constructing new layers.
.Sh INSTANTIATING NEW NULL LAYERS
New null layers are created with
takes two arguments, the pathname
of the lower vfs (target-pn) and the pathname where the null
layer will appear in the namespace (mount-point-pn). After
the null layer is put into place, the contents
of target-pn subtree will be aliased under mount-point-pn.
.Sh OPERATION OF A NULL LAYER
The null layer is the minimum file system layer,
simply bypassing all possible operations to the lower layer
for processing there. The majority of its activity centers
on the bypass routine, though which nearly all vnode operations
The bypass routine accepts arbitrary vnode operations for
handling by the lower layer. It begins by examing vnode
operation arguments and replacing any null-nodes by their
lower-layer equivlants. It then invokes the operation
on the lower layer. Finally, it replaces the null-nodes
in the arguments and, if a vnode is return by the operation,
stacks a null-node on top of the returned vnode.
Although bypass handles most operations,
must change the fsid being returned.
and vop_reclaim are not bypassed so that
they can handle freeing null-layer specific data.
is not bypassed to avoid excessive debugging
.Sh INSTANTIATING VNODE STACKS
Mounting associates the null layer with a lower layer,
in effect stacking two VFSes. Vnode stacks are instead
created on demand as files are accessed.
The initial mount creates a single vnode stack for the
root of the new null layer. All other vnode stacks
are created as a result of vnode operations on
this or other null vnode stacks.
New vnode stacks come into existance as a result of
an operation which returns a vnode.
The bypass routine stacks a null-node above the new
vnode before returning it to the caller.
For example, imagine mounting a null layer with
.Bd -literal -offset indent
mount_null /usr/include /dev/layer/null
the root null-node (which was created when the null layer was mounted).
done on the root null-node. This operation would bypass through
to the lower layer which would return a vnode representing
Null_bypass then builds a null-node
and returns this to the caller.
Later operations on the null-node
process when constructing other vnode stacks.
.Sh CREATING OTHER FILE SYSTEM LAYERS
One of the easiest ways to construct new file system layers is to make
a copy of the null layer, rename all files and variables, and
then begin modifing the copy. Sed can be used to easily rename
The umap layer is an example of a layer descended from the
.Sh INVOKING OPERATIONS ON LOWER LAYERS
There are two techniques to invoke operations on a lower layer
when the operation cannot be completely bypassed. Each method
is appropriate in different situations. In both cases,
it is the responsibility of the aliasing layer to make
the operation arguments "correct" for the lower layer
by mapping an vnode arguments to the lower layer.
The first approach is to call the aliasing layer's bypass routine.
This method is most suitable when you wish to invoke the operation
currently being hanldled on the lower layer. It has the advantage
the the bypass routine already must do argument mapping.
A second approach is to directly invoked vnode operations on
The advantage of this method is that it is easy to invoke
arbitrary operations on the lower layer. The disadvantage
is that vnodes arguments must be manualy mapped.
UCLA Technical Report CSD-910056,
.Em "Stackable Layers: an Architecture for File System Development" .
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