Size IBM Z networking from measured workload demand and the capacity you need after a failure—not from a single adapter speed. First separate traffic that can stay inside the machine from traffic that must reach an external LAN; then select a supported path, plan for the actual failure domains, and verify the complete adapter-to-switch link configuration. IBM’s materials describe topology and compatibility choices, but do not provide a universal bandwidth formula or target utilization threshold.
What to measure before choosing bandwidth
Build a workload profile that distinguishes normal operation from bursts and failover. Average utilization alone can hide short peaks or the extra traffic that lands on surviving paths after a failure.
- Throughput: Record sustained and peak traffic, including direction and time of day where relevant.
- Latency sensitivity: Identify workloads for which added network delay affects service performance.
- Processor consumption: Track the processor cost associated with the network workload, not just bytes transferred.
- Concurrency and workload type: Note connection counts and whether the traffic comes from online transactions, database queries, file transfers, or another workload.
- Traffic locality: Separate communication that can remain within IBM Z from traffic that must leave the CPC for an external LAN.
- Failure demand: Estimate how much traffic surviving paths would carry if an adapter feature, cable, or upstream network path were unavailable.
IBM’s networking performance material uses latency, throughput, processor usage, connection scaling, and workload type as comparison dimensions. Its underlying presentation spans 2009–2018, so those dimensions are useful for framing measurements, not as current benchmarks or sizing values. The IBM guidance reviewed does not establish a universal IBM Z bandwidth equation or standard oversubscription ratio. Use observed telemetry and service requirements to make the capacity decision.
Choose a path that matches where the traffic goes
Network paths are not interchangeable. An internal networking choice, an external LAN adapter, and a guest virtual network solve different topology problems; virtual networking does not itself create additional physical uplink capacity.
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| Path class | When it may fit | Planning point |
|---|---|---|
| HiperSockets | Communication that can remain inside IBM Z. | IBM describes HiperSockets as internal networking that does not require network cards. It is not a substitute for a path to an external LAN. |
| OSA-Express or Network Express | External Ethernet LAN connectivity. | IBM documents OSA as an integrated LAN-connectivity adapter and Network Express as extending OSA technology. Verify support and behavior for the target machine and software level; z/VM documentation also calls out EQDIO considerations for Network Express. |
| RoCE and SMC-R | A workload and software stack that support the RDMA path and whose physical network is configured for it. | Confirm generation, operating-system, and network requirements. SMC-R still relies on traditional TCP/IP connectivity and the associated physical network configuration. |
| z/VM guest LANs and virtual switches | Guest-to-guest or guest-to-network connectivity in a z/VM environment. | Include guest LANs, VSWITCHes, HiperSockets, and physical uplinks in the topology. These are distinct design choices; a virtual switch does not add bandwidth to its physical uplink. |
Adapter family, generation, protocol, and software release constrain which paths are available. IBM documentation distinguishes adapter generations and QDIO/EQDIO behavior; a stated link rate alone is not a recommendation for a workload. Confirm the supported configuration for the exact IBM Z model and operating-system or hypervisor release before choosing.
Design redundancy around the failures you need to survive
Start by naming the failure events the service must tolerate. Redundancy is useful only when the alternate path avoids the relevant failed component and the workload can actually use it.
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- Set the resilience objective. Decide whether the design must withstand loss of a port, adapter feature, cable, switch, or physical network.
- Map dependencies end to end. Trace each path through adapter features, cables, switch ports, switches, VLANs, and upstream routing. Two connections that share a required component do not protect against that component’s failure.
- Check alternate-path operation. Verify that routing, VLAN configuration, and workload failover direct traffic over the surviving path when a failure occurs.
- Size for the degraded state. Model the traffic on remaining paths after a loss, as well as normal traffic. Do not assume redundant paths automatically balance load or provide usable capacity equal to the sum of their link rates.
- Recheck protocol-specific requirements. Match the resilience design to the applicable adapter, physical-network, and software requirements.
IBM’s LinuxONE site-planning checklist states, “IBM recommends network connections are defined with redundancy.” For SMC-R with Network Express in the documented z/OS 3.2 and IBM z17-or-later context, IBM says to provide two Network Express features per unique physical network. Treat that as context-specific guidance, not a universal count for every IBM Z design; verify the requirement for the target machine and software release.
Keep multipath interfaces and link settings compatible
More paths are not automatically better if their speeds or physical settings are incompatible with the intended configuration.
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- Avoid mixed-speed interfaces in specified groups. IBM warns against mixing different-speed OSA interfaces in the same multipath group or LAN group, and in certain Sysplex Distributor arrangements, particularly under heavy workload. Check the applicable configuration guidance before combining interfaces.
- Verify FEC for applicable 25 GbE links. IBM states that Reed-Solomon FEC is required for IBM Z 25 GbE OSA and RoCE. Its site-planning checklist calls out the RS-IEEE FEC setting for Cisco connections. Confirm the setting against current IBM system documentation and the target switch vendor’s guidance.
- Check speed behavior across the entire link. IBM cautions that adapter optics operate at fixed speeds and, in the cited system context, do not auto-negotiate to slower rates. Confirm that adapter, optic, cable, and switch port are compatible at the intended speed rather than assuming a link will fall back.
Before ordering or commissioning, validate the exact adapter feature, generation, optics, cable, switch port, FEC mode, and software support together. Compatibility of an individual component does not establish compatibility of the complete path.
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Use the following sequence to arrive at a defensible design without applying an unsupported universal formula:
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- Group traffic by destination and workload. Separate internal communication from external LAN flows, and identify the workload types and service requirements associated with each.
- Establish observed demand. Use sustained and peak throughput, latency, processor consumption, and concurrency measurements. Include bursts and relevant workload schedules.
- Choose supported path classes. Match internal traffic to an internal path where appropriate, and select external adapters or RDMA options only when the machine, software, workload, and network support them.
- Test normal and failure cases in the design. Determine what each surviving path must carry when another path or shared component is lost; account for routing and workload behavior, not just nominal link rates.
- Validate implementation details. Confirm adapter generations, QDIO/EQDIO behavior where relevant, speeds, FEC, optics, cabling, switch settings, and software-level requirements in current IBM and network-vendor documentation.
Review the plan when workload mix, peak concurrency, network topology, IBM Z generation, or software release changes. The IBM materials available for this topic do not publish customer-specific telemetry, a universal target utilization, or a generally applicable throughput figure, so a precise recommendation depends on measurements from the intended environment.
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