IBM Power Sizing Basics
Sizing an IBM Power system means matching processor CPW, memory, and storage to a workload, plus the correct IBM i software tier, with enough headroom for growth. This guide covers the basic concepts IT teams need before evaluating an AS400 or older Power hardware upgrade.
Sizing an IBM Power Systems purchase or upgrade means answering one core question: how much processor, memory, and storage capacity does this workload actually need, on this specific hardware generation, with enough room to grow before the next upgrade cycle? Undersizing leads to performance problems within a year or two of go-live; oversizing wastes budget and, on IBM i, can push a purchase into a higher and more expensive software tier than necessary. This guide covers the basic concepts, without replacing a formal sizing study for a specific environment.
Why IBM Power Sizing Matters
Unlike many x86 environments where adding capacity is a matter of provisioning another virtual machine, an IBM Power Systems purchase is a discrete hardware decision: a specific model, a specific number of active cores, a specific amount of memory, and a specific IBM i software tier, often locked in for five or more years. Getting the size wrong is expensive in both directions, which is why CPW-based sizing, rather than guesswork, is the standard approach for IBM i environments.
CPW and Processor Sizing
CPW (Commercial Processing Workload) is IBM's benchmark for comparing IBM i processing capacity across hardware generations and models. The basic sizing method is to establish the CPW rating of the current or comparable system, measure or estimate actual utilization against that rating, and then select a target system whose CPW rating covers current usage plus planned growth. CPW ratings vary not just by processor generation but by how many cores are active and by IBM i software tier, so two systems with the same processor chip can have very different usable CPW depending on configuration.
IBM i Software Tiers and Licensing
IBM i software is licensed by processor tier, not simply by core count. IBM groups systems into tiers such as P05, P10, P20, P30, P40, and P50, based on their maximum CPW rating, and IBM i licensing, along with much third-party IBM i software licensing, is priced against the tier rather than the raw hardware. This means a sizing decision is not just a hardware question: choosing a configuration that crosses a tier boundary can materially change the total cost of the project, sometimes more than the hardware cost difference itself.
Memory and Storage Sizing
Processor CPW gets the most attention, but memory and storage sizing matter just as much for real-world performance. IBM i memory sizing depends on the number of active jobs, database buffer requirements, and how much memory is reserved by system firmware versus available to IBM i itself, which varies by hardware generation. Storage sizing depends on database size, growth rate, and whether the environment uses IBM i auxiliary storage pools (ASPs) to separate journal receivers, system data, or specific applications onto dedicated storage.
Growth Headroom and Right-Sizing Methodology
A sizing exercise should not target 100% utilization of the new system on day one. Standard practice is to size to current peak utilization plus a growth allowance, commonly in the range of 20-30% additional headroom, adjusted for the organization's known growth plans and how long the hardware is expected to stay in service. IBM's Systems Workload Estimator and vendor-run performance sizing studies are the standard tools for a formal sizing exercise; this article covers the underlying concepts, not a substitute for that analysis on a specific environment. See the Power P05 generation comparison dataset for an example of generation-over-generation CPW comparisons used in sizing work.