A major issue for clusters is a failure that causes the cluster to become partitioned (called split brain). When split brain occurs, not all nodes can communicate, so individual nodes or subsets of nodes might try to form individual or subset clusters. Each subset or partition might “believe” it has sole access and ownership to the multihost devices. When multiple nodes attempt to write to the disks, data corruption can occur.
Failure fencing limits node access to multihost devices by physically preventing access to the disks. When a node leaves the cluster (it either fails or becomes partitioned), failure fencing ensures that the node can no longer access the disks. Only current member nodes have access to the disks, resulting in data integrity.
Disk device services provide failover capability for services that use multihost devices. When a cluster member that currently serves as the primary (owner) of the disk device group fails or becomes unreachable, a new primary is chosen. The new primary enables access to the disk device group to continue with only minor interruption. During this process, the old primary must forfeit access to the devices before the new primary can be started. However, when a member drops out of the cluster and becomes unreachable, the cluster cannot inform that node to release the devices for which it was the primary. Thus, you need a means to enable surviving members to take control of and access global devices from failed members.
The Sun Cluster system uses SCSI disk reservations to implement failure fencing. Using SCSI reservations, failed nodes are “fenced” away from the multihost devices, preventing them from accessing those disks.
SCSI-2 disk reservations support a form of reservations, which either grants access to all nodes attached to the disk (when no reservation is in place). Alternatively, access is restricted to a single node (the node that holds the reservation).
When a cluster member detects that another node is no longer communicating over the cluster interconnect, it initiates a failure fencing procedure to prevent the other node from accessing shared disks. When this failure fencing occurs, the fenced node panics with a “reservation conflict” message on its console.
The discovery that a node is no longer a cluster member, triggers a SCSI reservation on all the disks that are shared between this node and other nodes. The fenced node might not be “aware” that it is being fenced and if it tries to access one of the shared disks, it detects the reservation and panics.