HP Windows Server 2012 Cluster Pre-Planning Guide

3 Windows Server 2012 Failover Clustering Behavior
Microsoft Failover Clustering exclusively uses SCSI-3 Persistent Reservations. Microsoft Failover
Clustering generates a single unique reservation key per cluster node; the key is used for all LUNs
managed by the cluster. Prior to bringing a disk resource online and reserving the associated
logical unit (LUN), an initiator must register its reservation key with the LUN. The same eight-byte
reservation key may then be used by the initiator to reserve the LUN for shared or exclusive access.
The array keeps track of which initiators have access to a LUN using these unique reservation keys
sent from the members of the cluster. The behavior of Microsoft Failover Clustering differs depending
on whether the LUN is being managed as a cluster disk resource or a cluster shared volume.
Cluster Disk Resources
Clustering disk resources allows a single cluster node exclusive access to the resource. If a host is
to online and own the resource, Microsoft Failover Clustering registers each of the host’s logical
paths to the LUN. After registration has occurred, Microsoft Failover Clustering sends a reservation
down a single logical path to reserve the LUN. Although there can be multiple registrations, there
can be only a single reservation per LUN. The reservation will be of type “Write Exclusive –
Registrants Only”, which allows access on any path (ITL nexus) registered with the LUN. For
example, if the host has four paths, then there will be four identical keys registered for that LUN.
After registering the nodes unique key on every path, a single reservation is sent using exactly the
same key. The reservation type allows the reserved LUN to be accessed on any of the four registered
paths.
When releasing a reservation on a LUN, Microsoft Failover Clustering releases the reservation on
the LUN using a single path. At this point, the host no longer has exclusive access to the LUN. After
the reservation has been released, each logical path to that LUN is unregistered.
Figure 2 Cluster Disk Resource
6 Windows Server 2012 Failover Clustering Behavior