drbd (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.

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 toi edit either
the KERNEL (uname -r) or KERNELPATH (/lib/modules/2.6.x-foo/build)
environment variable to point to the correct source or headers.