Oracle 5.0 Reference Manual page 1345

Overview of MySQL with DRBD/Pacemaker/Corosync/Oracle Linux
At the lowest level, 2 hosts are required in order to provide physical redundancy; if using a virtual
environment, those 2 hosts should be on different physical machines. It is an important feature that no
shared storage is required. At any point in time, the services will be active on one host and in standby
mode on the other.
Pacemaker and Corosync combine to provide the clustering layer that sits between the services and
the underlying hosts and operating systems. Pacemaker is responsible for starting and stopping
services, ensuring that they are running on exactly one host, thus delivering high availability and
avoiding data corruption. Corosync provides the underlying messaging infrastructure between the
nodes that enables Pacemaker to do its job; it also handles the nodes membership within the cluster
and informs Pacemaker of any changes.
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