Multipath High-Availability Cabling - IBM N Series Hardware Manual

6.3 Multipath High-Availability cabling

A standard N series clustered storage system has multiple single-points-of-failure on each
shelf which can trigger a cluster failover (Example 6-1). Cluster failovers can disrupt access to
data and put an increased workload on the surviving cluster node.
Example 6-1 Clustered system with a single connection to disks
N6270A> storage show disk –p
PRIMARY PORT SECONDARY PORT SHELF BAY
------- ---- --------- ---- ---------
0a.16
0a.18
0a.19
0a.20
Multipath High-Availability (MPHA) cabling adds redundancy, reducing the number of
conditions that can trigger a failover (Example 6-2).
Example 6-2 Clustered system with MPHA connections to disks
N6270A> storage show disk -p
PRIMARY PORT SECONDARY PORT SHELF BAY
------- ---- --------- ---- ---------
0a.16
0c.17
0c.18
0a.19
With only a single connection to the A channel, a disk 'loop' is technically a daisy chain. When
any component (fiber cable, shelf cable, shelf controller) in the loop fails, access is lost to all
shelves 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 B
channel. Multiple shelves can experience failures without losing communication to the
controller. A cluster failover is only triggered when a single shelf experiences failures to both
the A and B channels.
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IBM System Storage N series Hardware Guide
A
A
A
A
A 0c.16 B
B 0a.17 A
B 0a.18 A
A 0c.19 B
1 0
1 2
1 3
1 4
1 0
1 1
1 2
1 3