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1================================
2kernel data structure for DRBD-9
3================================
4
5This describes the in kernel data structure for DRBD-9. Starting with
6Linux v3.14 we are reorganizing DRBD to use this data structure.
7
8Basic Data Structure
9====================
10
11A node has a number of DRBD resources. Each such resource has a number of
12devices (aka volumes) and connections to other nodes ("peer nodes"). Each DRBD
13device is represented by a block device locally.
14
15The DRBD objects are interconnected to form a matrix as depicted below; a
16drbd_peer_device object sits at each intersection between a drbd_device and a
17drbd_connection::
18
19 /--------------+---------------+.....+---------------\
20 | resource | device | | device |
21 +--------------+---------------+.....+---------------+
22 | connection | peer_device | | peer_device |
23 +--------------+---------------+.....+---------------+
24 : : : : :
25 : : : : :
26 +--------------+---------------+.....+---------------+
27 | connection | peer_device | | peer_device |
28 \--------------+---------------+.....+---------------/
29
30In this table, horizontally, devices can be accessed from resources by their
31volume number. Likewise, peer_devices can be accessed from connections by
32their volume number. Objects in the vertical direction are connected by double
33linked lists. There are back pointers from peer_devices to their connections a
34devices, and from connections and devices to their resource.
35
36All resources are in the drbd_resources double-linked list. In addition, all
37devices can be accessed by their minor device number via the drbd_devices idr.
38
39The drbd_resource, drbd_connection, and drbd_device objects are reference
40counted. The peer_device objects only serve to establish the links between
41devices and connections; their lifetime is determined by the lifetime of the
42device and connection which they reference.