Documentation/blockdev/drbd/data-structure-v9.txt

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