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MOUNT_NULL(8)           OpenBSD System Manager's Manual          MOUNT_NULL(8)

     mount_null - demonstrate the use of a null file system layer

     mount_null [-o options] target mount_point

     The mount_null command creates a null layer, duplicating a sub-tree of
     the file system namespace under another part of the global file system
     namespace.  It is implemented using a stackable layers technique, and its
     ``null-nodes'' stack above all lower-layer vnodes (not just above direc-
     tory vnodes).

     The options are as follows:

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

     The null layer has two purposes.  First, it serves as a demonstration of
     layering by proving a layer which does 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
     with a null layer.

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

     New null layers are created with mount_null.  mount_null takes two argu-
     ments: 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.

     The null layer is the minimum file system layer, simply bypassing all
     possible operations to the lower layer for processing there.  The majori-
     ty 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 re-
     placing 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.

     Mounting associates the null layer with a lower layer, in effect stacking
     two VFSes.  Vnode stacks are instead created on demand as files are ac-

     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 op-
     erations on 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 vn-
     ode before returning it to the caller.

     For example, imagine mounting a null layer with

           # mount_null /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

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

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

     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 any 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 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 vnodes arguments must be manually mapped.

     mount(2), mount(8), umount(8)

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

     The mount_null utility first appeared in 4.4BSD.

OpenBSD 3.6                     April 19, 1994                               2