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 FlashSystem, PBHA: Are dedicated private ISLs _required_?

Ralf Lichtenstein's profile image
Ralf Lichtenstein posted Wed December 04, 2024 09:05 AM

Hi community,

please allow for the following question.

Does PBHA require ("by code")  dedicted SAN ISLs? The ones that were _required_ with Hyperswap (via ISLs)...

Background: 
- The official docs do not state the ultimate requirement for "dediceted private ISLs for replication traffic". The docs state "Best practice is to ensure that cluster ports performing node to node traffic are separated from those performing inter-system replication, or host I/O" - there no hint on SAN ISLs. It is just about "the cluster ports". One _could_ argue: "we seperated the cluster ports (via zoning) and used ISLs for host (or storage or other) _and_ replication traffic. This is not prohibited by the 'law of requirements'"
- On the other side www.redbooks.ibm.com/abstracts/sg248569.html states:  "SAN-zoning requirements are the same as for HyperSwap configurations, which require dedicated ISL-connections for public traffic, which provides hosts communication, and for private traffic, which provides node-to-node communication." But... redbooks are not supposed to be official documentation.
- And... there might be customer environments where using "one" ISL (or one ISL trunk) (per redundant fabric) is more the sufficient. But is this configurtation _allowed_ and _supported_?

Uncertain regards

Ralf

Philip Clark's profile image
Philip Clark posted Thu December 05, 2024 07:58 AM

Dedicated ports should be used for HA traffic vs host I/O.

For a simple config with no ISLs, zoning is sufficient to achieve this. With ISLs, there are two approaches that are supported:

  • Dedicated private and public ISLs
  • Shared ISLs but using switch-based QoS to give a higher priority to HA traffic

On HyperSwap we previously required SCORE approval for using shared ISLs. SCORE is no longer required for PBHA, but the same best principles apply.

Ralf Lichtenstein's profile image
Ralf Lichtenstein posted Thu December 05, 2024 08:39 AM

Hi Philip,

thank you very much (again). I also appreciate that.

I came to the exact same result after talking to Level2/3 storage support and one of the redbook writers and some other IBMers and some of my SVA colleagues. I had to "re-assemble" the answers into ... what you wrote in you answer. I am a bit uncertain why this is not documented clearly in the offical documentation. hmm... 

Kind regards

Ralf

Philip Clark's profile image
Philip Clark posted Thu December 05, 2024 08:54 AM

8.7.0 Technical Update has the following note:

If sharing an ISL for node-to-node and host traffic, QoS should be configured to prioritise replication traffic.

But I agree it is not clear in the docs and I have let the team know.