MOUNT_NULL(8) BSD System Manager's Manual MOUNT_NULL(8)NAMEmount_null — mount a loopback filesystem sub-tree; demonstrate the use of
a null file system layer
SYNOPSISmount_null [-o options] target mount-point
mount_null-u [-o options] mount-point
DESCRIPTION
The mount_null 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.
The primary differences between a virtual copy of the filesystem and a
symbolic link are that the getcwd(3) functions work correctly in the vir‐
tual copy, and that other filesystems 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 null filesystem 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 direc‐
tory 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.
-u Update the mount point. This is typically used to upgrade a
mount to read-write or downgrade it to read-only.
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 HAMMER PFSs. Third, the
null layer can serve as a prototype layer. Since it provides all neces‐
sary 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_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.
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_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(5) sys (assuming that the lower layer is an UFS(5)
file system). Null_bypass then builds a null-node aliasing the UFS(5)
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. Sed(1) can be used to easily rename all variables.
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 ALSOHAMMER(5), 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. Matthew Dillon made
mount_null work in DragonFly 1.7, after it had been broken for some time.
BSD September 28, 2008 BSD
