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.

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.

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. The challenge is to make these two independent zones easy to understand and maintain. There are two fundamentally different types of zoning D,P and WWPN.

D,P zoning works by port numbers in SAN switches. D,P can be very simple to understand with a dual fabric SAN, if you use the same port numbers for each host and SVC node on the corresponding switches in each fabric. The result is that even though the switch numbers will be different, the port numbers are the same in each fabric, so the zoning should look the same. Ad advantage is that if you re-cable a device to a different port, then you need to update the zoning.

WWPN zoning uses the world wide port name to identify the HBA ports on the other ends of the cables, so it is independent of the way the devices are cables together. However if you need to swap out an HBA, then the WWPN name will probably change and so the zoning must be changed. You would normally use aliases to make the WWPNs more comprehensible, so if you do this, then you just need to change the alias. If you chose your aliases carefully, then that can make the dual fabric reasonably easy to follow. WWPN zoning is essential if you want to use virtualisation.

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