Tom Vogt has written a separate UDP HA heartbeat subsystem called Heart . This is the document which started the Linux-HA project. It is full of good thoughts about High-Availability systems. It was written by Harald Milz . You can find it at cvs.linux-ha.org .
More details are to be found in the the announcement email .
LAN Mirroring technologies are an important HA technology now, and will remain a key ingredient in many inexpensive HA systems. They allow two machines to mirror a filesystem between themselves with only two NICs and a crossover cable. It is RAID and LAN aware with a quick sync option for bringing the mirror up to date quickly. This is the technology which is currently being followed most closely by Linux-HA people. Jerome Etienne 's O nline D isk R eplicator proposal.