kernel data structure for DRBD-9 — The Linux Kernel 5.18.5-gnu-1 documentation

# kernel data structure for DRBD-9

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.