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MOUNT_NULLFS(8)		FreeBSD	System Manager's Manual	       MOUNT_NULLFS(8)

NAME
     mount_nullfs -- mount a loopback file system sub-tree; demonstrate	the
     use of a null file	system layer

SYNOPSIS
     mount_nullfs [-o options] target mount-point

DESCRIPTION
     The mount_nullfs utility 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.

     The primary differences between a virtual copy of the file	system and a
     symbolic link are that the	getcwd(3) functions work correctly in the vir-
     tual copy,	and that other file systems may	be mounted on the virtual copy
     without affecting the original.  A	different device number	for the	vir-
     tual copy is returned by stat(2), but in other respects it	is indistin-
     guishable from the	original.

     The mount_nullfs file system differs from a traditional loopback file
     system in two respects: it	is implemented using a stackable layers	tech-
     niques, and its ``null-node''s stack above	all lower-layer	vnodes,	not
     just over directory vnodes.

     The options are as	follows:

     -o	     Options are specified with	a -o flag followed by a	comma sepa-
	     rated string of options.  See the mount(8)	man page for possible
	     options and their meanings.

     The null layer has	two 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,	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.

     The remainder of this man page examines the null layer as a basis for
     constructing new layers.

INSTANTIATING NEW NULL LAYERS
     New null layers are created with mount_nullfs.  The mount_nullfs utility
     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.

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 major-
     ity of its	activity centers on the	bypass routine,	through	which nearly
     all vnode operations pass.

     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.

     Although bypass handles most operations, vop_getattr, vop_inactive,
     vop_reclaim, and vop_print	are not	bypassed.  Vop_getattr must change the
     fsid being	returned.  Vop_inactive	and vop_reclaim	are not	bypassed so
     that they can handle freeing null-layer specific data.  Vop_print is not
     bypassed to avoid excessive debugging information.

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	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.

     For example, imagine mounting a null layer	with

	   mount_nullfs	/usr/include /dev/layer/null

     Changing directory	to /dev/layer/null will	assign the root	null-node
     (which was	created	when the null layer was	mounted).  Now consider	open-
     ing sys.  A vop_lookup would be done on the root null-node.  This opera-
     tion would	bypass through to the lower layer which	would return a vnode
     representing the UFS sys.	Null_bypass then builds	a null-node aliasing
     the UFS sys and returns this to the caller.  Later	operations on the
     null-node sys will	repeat this process when constructing other vnode
     stacks.

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.  The sed(1) utility can be used to easily rename all
     variables.

     The umap layer is an example of a layer descended from the	null layer.

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.

     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
     null_getattrs in the null layer.

     A second approach is to directly invoke vnode operations on the lower
     layer with	the 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.

SEE ALSO
     mount(8)

     UCLA Technical Report CSD-910056, Stackable Layers: an Architecture for
     File System Development.

HISTORY
     The mount_nullfs utility first appeared in	4.4BSD.

FreeBSD	10.1			  May 1, 1995			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | INSTANTIATING NEW NULL LAYERS | OPERATION OF A NULL LAYER | INSTANTIATING VNODE STACKS | CREATING OTHER FILE SYSTEM LAYERS | INVOKING OPERATIONS ON LOWER LAYERS | SEE ALSO | HISTORY

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