1This describes the in kernel data structure for DRBD-9. Starting with
2Linux v3.14 we are reorganizing DRBD to use this data structure.
3
4Basic Data Structure
5====================
6
7A node has a number of DRBD resources.  Each such resource has a number of
8devices (aka volumes) and connections to other nodes ("peer nodes"). Each DRBD
9device is represented by a block device locally.
10
11The DRBD objects are interconnected to form a matrix as depicted below; a
12drbd_peer_device object sits at each intersection between a drbd_device and a
13drbd_connection:
14
15  /--------------+---------------+.....+---------------\
16  |   resource   |    device     |     |    device     |
17  +--------------+---------------+.....+---------------+
18  |  connection  |  peer_device  |     |  peer_device  |
19  +--------------+---------------+.....+---------------+
20  :              :               :     :               :
21  :              :               :     :               :
22  +--------------+---------------+.....+---------------+
23  |  connection  |  peer_device  |     |  peer_device  |
24  \--------------+---------------+.....+---------------/
25
26In this table, horizontally, devices can be accessed from resources by their
27volume number.  Likewise, peer_devices can be accessed from connections by
28their volume number.  Objects in the vertical direction are connected by double
29linked lists.  There are back pointers from peer_devices to their connections a
30devices, and from connections and devices to their resource.
31
32All resources are in the drbd_resources double-linked list.  In addition, all
33devices can be accessed by their minor device number via the drbd_devices idr.
34
35The drbd_resource, drbd_connection, and drbd_device objects are reference
36counted.  The peer_device objects only serve to establish the links between
37devices and connections; their lifetime is determined by the lifetime of the
38device and connection which they reference.
39