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