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.\"     @(#)mount_null.8	8.6 (Berkeley) 5/1/95
.\" $FreeBSD: src/sbin/mount_null/mount_null.8,v 1.11.2.6 2001/12/20 16:40:00 ru Exp $
.\" $DragonFly: src/sbin/mount_null/mount_null.8,v 1.8 2008/10/26 00:05:24 swildner Exp $
.\"
.Dd September 28, 2008
.Dt MOUNT_NULL 8
.Os
.Sh NAME
.Nm mount_null
.Nd "mount a loopback filesystem sub-tree; demonstrate the use of a null file system layer"
.Sh SYNOPSIS
.Nm
.Op Fl o Ar options
.Ar target
.Ar mount-point
.Nm
.Fl u
.Op Fl o Ar options
.Ar mount-point
.Sh DESCRIPTION
The
.Nm
command creates a
null layer, duplicating a sub-tree of the file system
name space under another part of the global file system namespace.
This allows existing files and directories to be accessed
using a different pathname.
.Pp
The primary differences between a virtual copy of the filesystem
and a symbolic link are that the
.Xr getcwd 3
functions work correctly in the virtual copy, and that other filesystems
may be mounted on the virtual copy without affecting the original.
A different device number for the virtual copy is returned by
.Xr stat 2 ,
but in other respects it is indistinguishable from the original.
.Pp
The
.Nm null
filesystem differs from a traditional
loopback file system in two respects: it is implemented using
a stackable layers techniques, and its
.Do null-node Dc Ns s
stack above
all lower-layer vnodes, not just over directory vnodes.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl o
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.It Fl u
Update the mount point.
This is typically used to upgrade a mount to
read-write or downgrade it to read-only.
.El
.Pp
The null layer has three purposes.
First, it serves as a demonstration of layering by providing a layer
which does nothing.
(It actually does everything the loopback file system does,
which is slightly more than nothing.)
Second, it is used for NFS exporting
.Nm HAMMER
PFSs.
Third, 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 by starting
with a null layer.
.Pp
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
.Nm .
.Nm Mount_null
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, through which nearly all vnode operations
pass.
.Pp
The bypass routine accepts arbitrary vnode operations for
handling by the lower layer.  It begins by examining vnode
operation arguments and replacing any null-nodes by their
lower-layer equivalents.  It then invokes the operation
on the lower layer.  Finally, it replaces the null-nodes
in the arguments and, if a vnode is returned by the operation,
stacks a null-node on top of the returned vnode.
.Pp
Although bypass handles most operations,
.Em vop_getattr ,
.Em vop_inactive ,
.Em vop_reclaim ,
and
.Em vop_print
are not bypassed.
.Em Vop_getattr
must change the fsid being returned.
.Em Vop_inactive
and
.Em vop_reclaim
are not bypassed so that
they can handle freeing null-layer specific data.
.Em Vop_print
is not bypassed to avoid excessive debugging
information.
.\"
.\"
.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.
.Pp
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.
.Pp
New vnode stacks come into existence 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.
.Pp
For example, imagine mounting a null layer with
.Bd -literal -offset indent
mount_null /usr/include /dev/layer/null
.Ed
.Pp
Changing directory to
.Pa /dev/layer/null
will assign
the root null-node (which was created when the null layer was mounted).
Now consider opening
.Pa sys .
A
.Em vop_lookup
would be
done on the root null-node.  This operation would bypass through
to the lower layer which would return a vnode representing
the
.Xr UFS 5
.Pa sys
(assuming that the lower layer is an
.Xr UFS 5
file system).
Null_bypass then builds a null-node
aliasing the
.Xr UFS 5
.Pa sys
and returns this to the caller.
Later operations on the null-node
.Pa sys
will repeat this
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 modifying the copy.
.Xr Sed 1
can be used to easily rename
all variables.
.\"
.\"
.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 a vnode argument to the lower layer.
.Pp
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 handled on the lower layer.
It has the advantage that
the bypass routine already must do argument mapping.
An example of this is
.Em null_getattrs
in the null layer.
.Pp
A second approach is to directly invoke vnode operations on
the lower layer with the
.Em VOP_OPERATIONNAME
interface.
The advantage of this method is that it is easy to invoke
arbitrary operations on the lower layer.  The disadvantage
is that vnode arguments must be manually mapped.
.\"
.\"
.Sh SEE ALSO
.Xr HAMMER 5 ,
.Xr mount 8
.Pp
UCLA Technical Report CSD-910056,
.Em "Stackable Layers: an Architecture for File System Development" .
.Sh HISTORY
The
.Nm
utility first appeared in
.Bx 4.4 .
.An Matthew Dillon
made
.Nm
work in
.Dx 1.7 ,
after it had been broken for some time.