Installation guide

Appendix A. The Device Mapper
92
The following example shows the command to list the dependencies of all currently configured
mapped devices.
[root@ask-07 ~]# dmsetup deps
testgfsvg-testgfslv3: 1 dependencies : (8, 16)
testgfsvg-testgfslv2: 1 dependencies : (8, 16)
testgfsvg-testgfslv1: 1 dependencies : (8, 16)
VolGroup00-LogVol01: 1 dependencies : (8, 2)
VolGroup00-LogVol00: 1 dependencies : (8, 2)
The following example shows the command to list the dependencies only of the device
lock_stress-grant--02.1722:
[root@grant-01 ~]# dmsetup deps lock_stress-grant--02.1722
3 dependencies : (253, 33) (253, 32) (253, 31)
A.3. Device Mapper Support for the udev Device Manager
The primary role of the udev device manager is to provide a dynamic way of setting up nodes
in the /dev directory. The creation of these nodes is directed by the application of udev rules in
userspace. These rules are processed on udev events sent from the kernel directly as a result of
adding, removing or changing particular devices. This provides a convenient and central mechanism
for hotplugging support.
Besides creating the actual nodes, the udev device manager is able to create symbolic links which the
user can name. This provides users the freedom to choose their own customized naming and directory
structure in the/dev directory, if needed.
Each udev event contains basic information about the device being processed, such as its name,
the subsystem it belongs to, the device's type, its major and minor number used, and the type of
the event. Given that, and having the possibility of accessing all the information found in the /sys
directory that is also accessible within udev rules, the users are able to utilize simple filters based on
this information and run the rules conditionally based on this information.
The udev device manager also provides a centralized way of setting up the nodes' permissions. A
user can easily add a customized set of rules to define the permissions for any device specified by any
bit of information that is available while processing the event.
It is also possible to add program hooks in udev rules directly. The udev device manager can call
these programs to provide further processing that is needed to handle the event. Also, the program
can export environment variables as a result of this processing. Any results given can be used further
in the rules as a supplementary source of information.
Any software using the udev library is able to receive and process udev events with all the
information that is available, so the processing is not bound to the udev daemon only.
A.3.1. udev Integration with the Device Mapper
In Red Hat Enterprise Linux 6, the Device Mapper provides direct support for udev integration. This
synchronizes the Device Mapper with all udev processing related to Device Mapper devices, including
LVM devices. The synchronization is needed since the rule application in the udev daemon is a form
of parallel processing with the program that is the source of the device's changes (such as dmsetup