In our Fibre Channel Zoning Basics webcast last month, FCIA experts Ed Mazurek and John Rodrigues, explained how and why zoning makes Fibre Channel so reliable, predictable and secure. If you missed the live presentation, you can watch it on-demand. As promised during the webcast, here are answers to the questions we received:
Q: For best practices, you mention that zoning should be done with PWWN or alias, but what would be a case that a user might want to zone with D,I?
A: Zoning with D,I requires fewer steps for an administrator to configure because they can enter in a couple D,I zone members and all the devices plugged into those D,I’s can communicate. You have to be careful with D,I members though because the domain ID for a switch can change which can affect connectivity if this does happen. This is typically seen more in smaller installations or FICON environments. But for the most part we recommend that users zone with Port WWN members.
Q: In your zone alias slide, you gave an example of two Hosts (or initiators) belonging to one alias, but this seems to go against the Single Initiator / Single Target scheme?
A: The example shown was to illustrate that multiple devices could belong to one alias. Typically you will see aliases containing one member or a host-target pair. But going back to the multi-host alias, this would be a valid/useful configuration to be used with Peer Zoning that contained an Alias as a peer member.
Q: For Peer Zoning, what commands in FC-GS are applicable in order to take advantage of it?
A: The Peer Zoning related commands in FC-GS-7 and newer releases are the AAPZ (Add/Replace Active Peer Zone), RAPZ (Remove Active Peer Zone), and GAPZ (Get Active Peer Zone) commands. You can find these in the ‘Fabric Zone Server – Enhanced Zoning Request Command Codes‘ table and throughout the document for more information. There is mention of Peer Zoning in FC-SW-7 as well.
Q: In the Peer Zones slide, there is mention of a 1-to-Many zoning scheme. What is this?
A: A 1-to-Many zoning scheme is when a user zones all devices in one zone. The typical strategy is zoning 1 target to many hosts (hence the name 1-to-Many). This is easier for an administrator to configure but results in unnecessary ACL host-to-host entries and can result in increased RSCN/NS-Queries on zoning changes. The picture on slide 32 with 12 initiators and 2 targets is an example of a 1-to-many zone.
Q: During the zoneset activation example, what would happen if a switch joins or leaves the fabric in the middle of an activation?
A: In the case of a switch joining the fabric, the joining switch would not participate in the Activation sequence. Typically, what would happen is the joining switch would merge in with the fabric, but the merge would have to wait until the activation sequence completed.
Q: Are RSCNs by subscription or are they required to be received?
A: RSCN’s are by subscription(registration) only. Each device must send a State Change Registration (SCR) into the switch to request the type of RSCNs they would like to receive.
Q: Is there a case where you would want to suppress RSCNs?
A: Maybe… If servers are operating in a clustered environment then perhaps only a subset of them require RSCNs.
Q: Is it possible to do fabric assigned World Wide Name (WWN)?
A: You Can use the FWWN, which is the WWN of the port in the switch, to do zoning. This would allow *any* devices connected to that port to be zoned with the other members of the zone. It also means that for ports/interfaces that have multiple logins (FLOGIs) all of those would be zoned to each other. This is not normally what is desired so for these and other reasons, this is not a preferable zone member type.
Q: Can I do my zoning before my servers arrive (with HBAs installed)?
A: Sure… If you know the WWNs then you can create alias names and use those alias names in the zones. A active zoneset can have zones in it where the members are not logged into the fabric. That is fine. Once they do login the zoning will take effect.
Q: What are the most common FC zoning parameters that should be looked at first to improve performance?”
A: For performance of devices, zoning and performance are orthogonal. Good zoning or bad zoning has no impact on performance. Performance is typically a function of the utilization of the storage array port. If you have too many devices accessing a storage port you may see performance issues. But to fix this, you need to move hosts around the different storage array ports and do the appropriate LUN masking. Of course, you’ll have to change your zones to match the new layout, but the zoning itself is not part of the performance improvement.
Now, having said that, one of the reasons why it’s considered best practice to have single initiator zoning (or, at the very least, limited devices in a zone), has to do with the RSCN interruptions that can occur. Whenever a device is notified of a state change, it can interrupt the I/O as the device registers a change in the fabric. This can be an undesirable situation that affects performance. This is a completely different question, however, than whether or not zoning “improves” performance, technically speaking.
Note: Special thanks to Mark Allen from Cisco for helping to provide input on some of these answers