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