NAME    [Toc]    [Back]

     mount_null - demonstrate the use of a null file system layer

SYNOPSIS    [Toc]    [Back]

     mount_null [-o options] target mount_point

DESCRIPTION    [Toc]    [Back]

     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 directory
 vnodes).

     The options are as follows:

     -o options
             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 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.

INSTANTIATING NEW NULL LAYERS    [Toc]    [Back]

     New null layers are  created  with  mount_null.   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-pointpn).  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    [Toc]    [Back]

     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    [Toc]    [Back]

     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 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_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 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  nullnode 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    [Toc]    [Back]

     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.

INVOKING OPERATIONS ON LOWER LAYERS    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

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

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

HISTORY    [Toc]    [Back]

     The mount_null utility first appeared in 4.4BSD.

OpenBSD     3.6                          April      19,      1994