What are Tie-breakers in Red Hat Cluster?
  • Tie-breakers are additional heuristics that allow a cluster partition to decide whether or not it is quorate in the event of an even-split - prior to fencing. 
  • With such a tie-breaker, nodes not only monitor each other, but also an upstream router that is on the same path as cluster communications. If the two nodes lose contact with each other, the one that wins is the one that can still ping the upstream router.That is why, even when using tie-breakers, it is important to ensure that fencing is configured correctly.
  • CMAN has no internal tie-breakers for various reasons. However, tie-breakers can be implemented using the API.
What is fencing in Red Hat Cluster?
  • Fencing is the disconnection of a node from the cluster's shared storage. 
  • Fencing cuts off I/O from shared storage, thus ensuring data integrity. 
  • The cluster infrastructure performs fencing through the fence daemon, fenced.
  • When CMAN determines that a node has failed, it communicates to other cluster-infrastructure components that the node has failed. 
  • fenced, when notified of the failure, fences the failed node. 
What are the various types of fencing supported by High Availability Add On?
  • Power fencing — A fencing method that uses a power controller to power off an inoperable node.
  • storage fencing — A fencing method that disables the Fibre Channel port that connects storage to an inoperable node.
  • Other fencing — Several other fencing methods that disable I/O or power of an inoperable node, including IBM Bladecenters, PAP, DRAC/MC, HP ILO, IPMI, IBM RSA II, and others.
What are the lock states in Red Hat Cluster?
A lock state indicates the current status of a lock request. A lock is always in one of three states:
Granted — The lock request succeeded and attained the requested mode.
Converting — A client attempted to change the lock mode and the new mode is incompatible with an existing lock.
Blocked — The request for a new lock could not be granted because conflicting locks exist.
A lock's state is determined by its requested mode and the modes of the other locks on the same resource.

What is DLM lock model?
  • DLM is a short abbreviation for Distributed Lock Manager.
  • A lock manager is a traffic cop who controls access to resources in the cluster, such as access to a GFS file system.
  • GFS2 uses locks from the lock manager to synchronize access to file system metadata (on shared storage)
  • CLVM uses locks from the lock manager to synchronize updates to LVM volumes and volume groups (also on shared storage)
  • In addition, rgmanager uses DLM to synchronize service states.
  • without a lock manager, there would be no control over access to your shared storage, and the nodes in the cluster would corrupt each other's data.