The High Availability cluster stack's highest priority is protecting the integrity of data. This is achieved by preventing uncoordinated concurrent access to data storage: For example, Ext3 file systems are only mounted once in the cluster, OCFS2 volumes will not be mounted unless coordination with other cluster nodes is available. In a well-functioning cluster Pacemaker will detect if resources are active beyond their concurrency limits and initiate recovery. Furthermore, its policy engine will never exceed these limitations.
However, network partitioning or software malfunction could potentially cause scenarios where several coordinators are elected. If this so-called split brain scenarios were allowed to unfold, data corruption might occur. Hence, several layers of protection have been added to the cluster stack to mitigate this.
The primary component contributing to this goal is IO fencing/STONITH since it ensures that all other access prior to storage activation is terminated. Other mechanisms are cLVM2 exclusive activation or OCFS2 file locking support to protect your system against administrative or application faults. Combined appropriately for your setup, these can reliably prevent split brain scenarios from causing harm.
This chapter describes an IO fencing mechanism that leverages the storage itself, followed by the description of an additional layer of protection to ensure exclusive storage access. These two mechanisms can be combined for higher levels of protection.