DRBD refers to block devices designed as a building block to form high availability (HA) clusters.Also possible to combine multiple devices driver for Linux(MD) and the network block device(NBD) to achieve similar functionality.

DRBD is done by mirroring a whole block device via an assigned network. DRBD can be understood as network based raid-1. Only two nodes are supported in 8.4.x. For the drbd9(dev in RC phase), multiple nodes can be supported as well. Diving into it and do hacks.