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
     virtual copy, and that other file systems may be mounted on the virtual
     copy without affecting the original.  A different device number for the
     virtual copy is returned by stat(2), but in other respects it is
     indistinguishable 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
     techniques, 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
             separated 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
     majority 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
     opening sys.  A 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 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_null utility first appeared in 4.4BSD.  It was renamed to
     mount_nullfs in FreeBSD 5.0.

FreeBSD 13.1-RELEASE-p6         October 3, 2016        FreeBSD 13.1-RELEASE-p6