SVC Zoning and Multi-pathing

IBM recommends that the SAN infrastructure containing an SVC is carved into three types of zone, Host zones, SVC node zones and Storage Zones. IBM also recommends that an SVC be connected to Hosts and Storage with a dual-fabric SAN for resilience. Tip - if you get any problems with your SVC SAN zoning you can use the lsfabric command to display the connectivity between nodes and other controllers and hosts.

The SVC node zones.

The SVC node zone should contain the Master Console and all the SVC nodes. An SVC node has two HBAs and each HBA has two fiber ports; the Master Console has two fiber ports. A dual fabric configuration should be zoned with two zones so that each Master Console port is zoned to one port on each HBA on every SVC cluster node as shown below. You should create one zone per fabric with all of the SVC ports cabled to this fabric to allow SVC internode communication.

Note that the SVC zone does not connect to any hosts or storage.

The Storage Zones

SVC rules say that an SVC node must be able to see the exact same HBA ports and Storage Subsystem ports as all the other SVC nodes in the cluster. The rules also say that the storage zone must not include any host ports. You can either have one storage zone that includes all the storage devices, or you can define a separate zone for each storage subsystem. The diagram below shows two storage subsystems with their own zones, each with two paths to the SVC for resilience.

The Host Zones.

Hosts and Storage devices usually connect together with more than one fiber channel port. A Host will see several instances of a LUN when this happens. For example, a host with a 2 port HBA that is connecting to a LUN with a single port will see two instances of that LUN. If the Host is connecting through 2 HBAs, each with dual ports, and the storage device has two ports, then the host will see 8 instances of a LUN. Hosts must be able to recognize that these multiple instances are the same LUN, to prevent data corruption and permit path failover. They do this by means of a Subsystem Device Driver (SDD), which combines those multiple instances into a single instance.
A Host will see the VDisks on an SVC as LUNs. Each VDisk is hosted on a single I/O group, and each of the 2 SVC nodes in the I/O group can have 4 ports. As the number of actual paths and instances must be 8 or less you must use no more than 4 Host ports to connect to two SVC ports. Incidentally ISL links between switches will increase the number of potential paths though a SAN, but they do not increase the number of LUN images. It is the combinations of host and SVC port pairs that counts, not the paths through the SAN.

SDD path failure will always use the preferred node unless ALL the paths to the preferred node have failed, in which case it will use the alternate node. The SVC will automatically fail back to the preferred node when the paths are reinstated

It is common practice that every Host be zoned independently to the SVC, and if you are using a dual fabric SAN, then you should define two zones for every host.

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