================================kernel data structure for DRBD-9================================ This describes the in kernel data structure for DRBD-9. Starting withLinux 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 ofdevices (aka volumes) and connections to other nodes ("peer nodes"). Each DRBDdevice is represented by a block device locally. The DRBD objects are interconnected to form a matrix as depicted below; adrbd_peer_device object sits at each intersection between a drbd_device and adrbd_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 theirvolume number.  Likewise, peer_devices can be accessed from connections bytheir volume number.  Objects in the vertical direction are connected by doublelinked lists.  There are back pointers from peer_devices to their connections adevices, and from connections and devices to their resource. All resources are in the drbd_resources double-linked list.  In addition, alldevices can be accessed by their minor device number via the drbd_devices idr. The drbd_resource, drbd_connection, and drbd_device objects are referencecounted.  The peer_device objects only serve to establish the links betweendevices and connections; their lifetime is determined by the lifetime of thedevice and connection which they reference.