68 IBM System Storage N series Hardware Guide6.3 Multipath High-Availability cablingA standard N series clustered storage system has multiple single-points-of-failure on eachshelf which can trigger a cluster failover (Example 6-1). Cluster failovers can disrupt access todata and put an increased workload on the surviving cluster node.Example 6-1 Clustered system with a single connection to disksN6270A> storage show disk –pPRIMARY PORT SECONDARY PORT SHELF BAY------- ---- --------- ---- ---------0a.16 A 1 00a.18 A 1 20a.19 A 1 30a.20 A 1 4Multipath High-Availability (MPHA) cabling adds redundancy, reducing the number ofconditions that can trigger a failover (Example 6-2).Example 6-2 Clustered system with MPHA connections to disksN6270A> storage show disk -pPRIMARY PORT SECONDARY PORT SHELF BAY------- ---- --------- ---- ---------0a.16 A 0c.16 B 1 00c.17 B 0a.17 A 1 10c.18 B 0a.18 A 1 20a.19 A 0c.19 B 1 3With only a single connection to the A channel, a disk ‘loop’ is technically a daisy chain. Whenany component (fiber cable, shelf cable, shelf controller) in the loop fails, access is lost to allshelves after the break, triggering a cluster failover event.MPHA cabling creates a true loop by providing a path into the A channel and out of the Bchannel. Multiple shelves can experience failures without losing communication to thecontroller. A cluster failover is only triggered when a single shelf experiences failures to boththe A and B channels.
