This describes the in kernel data structure for DRBD-9. Starting with
Linux v3.14 we are reorganizing DRBD to use this data structure.

Basic Data Structure
====================

A node has a number of DRBD resources.  Each such resource has a number of
devices (aka volumes) and connections to other nodes ("peer nodes"). Each DRBD
device is represented by a block device locally.

The DRBD objects are interconnected to form a matrix as depicted below; a
drbd_peer_device object sits at each intersection between a drbd_device and a
drbd_connection:

  /--------------+---------------+.....+---------------\
  |   resource   |    device     |     |    device     |
  +--------------+---------------+.....+---------------+
  |  connection  |  peer_device  |     |  peer_device  |
  +--------------+---------------+.....+---------------+
  :              :               :     :               :
  :              :               :     :               :
  +--------------+---------------+.....+---------------+
  |  connection  |  peer_device  |     |  peer_device  |
  \--------------+---------------+.....+---------------/

In this table, horizontally, devices can be accessed from resources by their
volume number.  Likewise, peer_devices can be accessed from connections by
their volume number.  Objects in the vertical direction are connected by double
linked lists.  There are back pointers from peer_devices to their connections a
devices, and from connections and devices to their resource.

All resources are in the drbd_resources double-linked list.  In addition, all
devices can be accessed by their minor device number via the drbd_devices idr.

The drbd_resource, drbd_connection, and drbd_device objects are reference
counted.  The peer_device objects only serve to establish the links between
devices and connections; their lifetime is determined by the lifetime of the
device and connection which they reference.