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Heartbeat's Linux-HA.org and Clusterlabs.org
Heartbeat is the cluster manager that is used with most DRBD® installations, the → original heartbeat site. The new resource manager of Heartbeat is now called Pacemaker and has its own → pacemaker website.
→ Csync2 is a very useful tool to keep files or whole directory trees in sync over two or more machines. It was initially designed to keep DRBD's and Heartbeat's configuration files in sync but it can do a lot more now.
GFS on DRBD Cookbook
→ At RedHat's Sourceware site there is a nice cookbook on how to use GFS on top of DRBD®.