LEARNING - Question of the WEEK - IBM Storage - DS4000 & ERM

Question of the week:

I was having a look through our Redbook and chapter 2.4.3 seems to indicate two connections are required (one from each controller on the primary storage subsystem to each controller on the secondary storage subsystem):

DS4000 Best Practices and Performance Tuning Guide

Is this a requirement that will set off alarms if not met or simply best practice?

If there is only a single FC connection between sites, will only LUNs with controller A as primary get mirrored?

Would there be any error indicators with only a single connection?

Answer of the week:

The way that ERM works, the A controller will mirror to the remote systems A controller via it’s high-end port, and the B controller will communicate with the remote systems B controller through its high-end port. If a local lun that’s being mirrored is moved to the secondary controller, the remote volume will also be failed over. So, the requirement to remotely connect both controllers in an ERM configuration is required, not just best practice. As far as only having one connection between sites, that is probably very common. Another requirement of ERM is the use of switches in the environment (do direct cross-over cable connection between controllers). With the switch (and/or multi-protocol routers) in the environment, both controllers can share the single inter-switch link (ISL) between sites. This is how almost all ERM environments are configured.

I do believe error messages would be generated if you tried to configure ERM with just a single controller having connectivity, but I’m not sure how successful configuring the environment would be (i.e. if it would make you correct the problem before continuing). Either way, this would probably fall under the “worst practices” category.