man: More /dev/vg and /dev/mapper documentation.

man/dmsetup.8.in

@@ -502,7 +502,13 @@ Sets the device geometry to C/H/S.
 .RI [ subsystem ]
 .br
 Splits given device name into subsystem constituents.
-Default subsystem is LVM.
+The default subsystem is LVM.  
+LVM currently generates device names by concatenating the names of the Volume
+Group, Logical Volume and any internal Layer with a hyphen as separator.
+Any hyphens within the names are doubled to escape them.
+The precise encoding might change without notice in any future
+release, so we recommend you always decode using the current version of
+this command.
 .br
 .TP
 .B status

man/lvchange.8.in

@@ -72,7 +72,18 @@ See \fBlvm\fP(8) for common options.
 Controls the availability of the logical volumes for use.
 Communicates with the kernel device-mapper driver via
 libdevmapper to activate (\-ay) or deactivate (\-an) the
-logical volumes. If autoactivation option is used (\-aay),
+logical volumes. 
+.IP
+Activation of a logical volume creates a symbolic link
+/dev/VolumeGroupName/LogicalVolumeName pointing to the device node.
+This link is removed on deactivation.
+All software and scripts should access the device through
+this symbolic link and present this as the name of the device.
+The location and name of the underlying device node may depend on 
+the distribution and configuration (e.g. udev) and might change 
+from release to release.
+.IP
+If autoactivation option is used (\-aay),
 the logical volume is activated only if it matches an item in
 the activation/auto_activation_volume_list set in lvm.conf.
 If this list is not set, then all volumes are considered for

man/lvm.8.in

@@ -347,6 +347,14 @@ There are also various reserved names that are used internally by lvm that can n
 A VG cannot be called anything that exists in /dev/ at the time of creation, nor can it be called '.' or '..'.
 A LV cannot be called '.' '..' 'snapshot' or 'pvmove'. The LV name may also not contain
 the strings '_mlog', '_mimage', '_rimage', '_tdata', '_tmeta'.
+A directory bearing the name of each Volume Group is created under /dev when any of its Logical Volumes are activated.
+Each active Logical Volume is accessible from this directory as a symbolic link leading to a device node.
+Links or nodes in /dev/mapper are intended only for internal use and the precise format and escaping might change between releases and distributions.
+Other software and scripts should use the
+/dev/VolumeGroupName/LogicalVolumeName format to reduce the chance of needing
+amendment when the software is updated.  Should you need to process the node
+names in /dev/mapper, you may use \fBdmsetup splitname\fP to separate out the
+original VG, LV and internal layer names.
 .SH ALLOCATION
 When an operation needs to allocate Physical Extents for one or more
 Logical Volumes, the tools proceed as follows:
@@ -500,6 +508,7 @@ All tools return a status code of zero on success or non-zero on failure.
 .BR lvmthin (7),
 .BR lvm\ dumpconfig (8),
 .BR clvmd (8),
+.BR dmsetup (8),
 .BR lvchange (8),
 .BR lvcreate (8),
 .BR lvdisplay (8),

man/vgchange.8.in

@@ -83,6 +83,15 @@ The \-aay option should be also used during system boot so it's
 possible to select which volumes to activate using the
 activation/auto_activation_volume_list settting.
 .IP
+Activation of a logical volume creates a symbolic link 
+/dev/VolumeGroupName/LogicalVolumeName pointing to the device node.
+This link is removed on deactivation.
+All software and scripts should access the device through
+this symbolic link and present this as the name of the device.
+The location and name of the underlying device node may depend on   
+the distribution and configuration (e.g. udev) and might change 
+from release to release.
+.IP
 If clustered locking is enabled, add 'e' to activate/deactivate
 exclusively on one node or 'l' to activate/deactivate only
 on the local node.