From ClusterLabs
Pacemaker achieves maximum availability for your cluster services by detecting and recovering from node and service-level failures.
It achieves this by utilizing the messaging and membership capabilities provided by your preferred cluster infrastructure (currently either Corosync or Heartbeat),
If the startup and shutdown of your service can scripted, Pacemaker can improve its availability.
Pacemaker can manage clusters of practically any size and comes with a powerful dependency model for accurately modeling your environment.
Common node configurations that are possible to configure with Pacemaker.
 Two-node Active/Passive clusters using Pacemaker and DRBD are a cost-effective solution for many High Availability situations |  By supporting many nodes, Pacemaker can dramatically reduce hardware costs by allowing several active/passive clusters to be combined and share a common backup node |
 When shared storage is available, every node can potentially be used for failover. Pacemaker can even run multiple copies of services to spread out the workload. |  Pacemaker 1.2 will include enhancements to simplify the creation of split-site clusters |
