HP-UX HB v13.00 Ch-15 - Serviceguard

HP-UX Handbook – Rev 13.00 Page 88 (of 108)
Chapter 15 Serviceguard
October 29, 2013
16. Read and adhere to the Special Installation Instructions of the Serviceguard patch you are
using.
Cluster Quorum to Prevent Split-Brain Syndrome (See the section
titled “Quorum Rules and Cluster Arbitration Device”)
In general, the algorithm for cluster re-formation requires a cluster quorum of a strict majority
(that is, more than 50%) of the nodes previously running. If both halves (exactly 50%) of a
previously running cluster were allowed to re-form, there would be a split-brain situation in
which two instances of the same cluster were running. In a split-brain scenario, different
incarnations of an application could end up simultaneously accessing the same disks.
Serviceguard’s quorum requirement is designed to prevent a split-brain situation.
Cluster Lock
The prevention of split-brain syndrome is guaranteed by the use of a tie-breaker to choose
between the two equal-sized node groups, allowing one group to form the cluster and forcing the
other group to shut down. This tie-breaker is known as a cluster lock. The cluster lock is
implemented by means of a lock disk, lock LUN or a quorum server. The cluster lock is used
as a tie-breaker only for situations in which a running cluster fails and, as Serviceguard attempts
to form a new cluster, the cluster is split into two sub-clusters of equal size. If you have a two
node cluster and the communications are lost between these two nodes, the node that obtains the
cluster lock will take over the cluster and the other node will halt or perform a TOC. Without a
cluster lock, a failure of either node in the cluster will cause the other node, and therefore the
cluster, to halt. Note also that if the cluster lock fails during an attempt to acquire it, the cluster
will halt.
Lock Requirements
The following table gives an overview which type of cluster has to or must not use a cluster lock
and which type of cluster lock is supported.
Type of Cluster
Cluster lock
Type of Cluster Lock
Lock Disk
Quorum Server
1 node cluster
Not required
N/A
N/A
2 node cluster
Required
Ok
Ok
3 and 4 node cluster
Recommended
Ok
Ok
> 4 node cluster
SCSI limited to 4
controllers, so no VG
Not Ok
Ok
You may consider using no cluster lock with configurations of three or more nodes, although the
decision should be affected by the fact that any cluster may require tie-breaking.
For Lock Disks you can choose between options—a single or dual lock disk—for further details
refer to the Managing Serviceguard manual.