drbd-kernel (Kernel Module for Distributed Replicated Block Device)

DRBD is a block device which is designed to build high availability clusters. 
This is done by mirroring a whole block device via (a dedicated) network. 
You could see it as a network RAID1. DRBD takes over the data, writes it to 
the local disk and sends it to the other host. On the other host, it takes it 
to the disk there.

Starting with Linux 2.6.33, DRBD is officially integrated into the vanilla 
Linux kernel source. Besides the kernel integration, DRBD continues to be 
developed out of tree.

If you have a 2.6.33 kernel, you should use the DRBD module shipped with 
your kernel, or compile it from your kernel sources if it is not available 
in your current configuration.

If you have an earlier kernel version then DRBD can be built as a module, 
or it can be patched into the kernel. This build creates a kernel module 
based on your current kernel using /lib/modules/$(uname -r)/build as the 
kernel source tree. This may not be appropriate for you and you may need to 
edit either KERNEL (uname -r) or KERNELPATH (/lib/modules/2.6.x-foo/build) 
environment variable to point to the correct source or headers.