Hi Sebastian,
For LinuxONE environments, even though storage supports direct host attachment, I would strongly recommend using a SAN fabric instead of direct connectivity - especially in a shared storage design between multiple systems.
There are a few key reasons behind this recommendation:
- Observability & Diagnostics: With SAN switches, you gain end-to-end visibility of the I/O path. This includes buffer credits, latency indicators, congestion, and error counters. In direct attach scenarios, this layer is completely missing, you are effectively blind between host and storage.
- Operational Risk in Mission-Critical Systems: LinuxONE is typically positioned for highly critical workloads. In such environments, even low-level signal degradation (optical TX/RX issues etc.) must be detectable centrally. SAN fabrics allow proactive alerting and correlation, which is not possible with direct connections.
- Scalability & Multi-host Design: While technically multiple hosts can be connected directly, scaling beyond simple topologies becomes complex and operationally fragile. A SAN fabric provides clean zoning, isolation, and predictable expansion when adding more hosts or storage.
- Resiliency & Path Management: SAN-based designs inherently support better path diversity and failure domain separation. Direct attach limits your ability to design true fabric-level redundancy.
Direct host connectivity may work from a purely functional standpoint, but for LinuxONE-class workloads, it introduces operational blind spots and limits scalability. For that reason, a SAN-based architecture is the more robust and supportable design.
In short, direct attach works, until the day you actually need to troubleshoot something!
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Nezih Boyacioglu
IBM Community Hero
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