① Explainer · Licensing concepts
IBM Z software pricing has moved through three eras: full capacity metrics like PSLC, sub-capacity metrics like WLC and AWLC, and consumption based Tailored Fit Pricing. Most estates run a mix of all three. Here is the timeline, what each model bills, and why knowing which metric applies to which product is the start of any optimization.
Three eras, running side by side.
IBM Z software pricing has evolved from charging for the whole machine to charging for the measured peak to charging for actual consumption. Parallel Sysplex License Charges, PSLC, appeared in the 1990s as a full capacity metric: the bill followed the total rated MSU of the machine. Workload License Charges, WLC, arrived in 2000 and introduced sub-capacity, billing the Rolling 4-Hour Average peak of the logical partitions running each product. Advanced Workload License Charges, AWLC, and the entry level variants refined sub-capacity across later machine generations. Tailored Fit Pricing, TFP, introduced in 2019, shifted again, to billing actual annual MSU consumption against a baseline.
The reason this history matters is that the eras did not replace each other cleanly. Most estates run a mix of metrics across their products, and each metric has different levers. Peak shaping moves a sub-capacity WLC bill but does nothing for a full capacity PSLC product. A TFP container is governed by its baseline, not its monthly peak. Mapping which metric applies to which product is the first step of any review, which is why this explainer feeds directly into our MSU optimization service.
From full capacity to consumption, the order of arrival
| Era | Metric | Bills on | Buyer lever |
|---|---|---|---|
| 1990s | PSLC, full capacity | Total rated MSU of the machine | Machine size or a metric move; peak shaping does nothing |
| 2000 | WLC, sub-capacity | R4HA peak of the LPARs running the product | Peak shaping, capping, zIIP offload, accurate SCRT |
| 2011 | IWP, integrated workload | Aggregated qualifying workload within sub-capacity | Workload mix and placement |
| z196 era onward | AWLC and entry variants | R4HA peak, aligned to newer machine generations | Peak shaping plus hardware refresh timing |
| 2019 | Tailored Fit Pricing | Actual annual MSU consumption against a baseline | Baseline accuracy and growth terms |
Same MSU, different game.
The three eras do not just bill differently; they reward different behavior. The illustration below indexes a single 500 MSU workload across the three pricing philosophies to show how the billable basis changes even when the work does not. Under full capacity the bill follows the 1,000 MSU machine; under sub-capacity it follows the 500 MSU peak; under consumption it follows the actual MSU used across the year against the agreed baseline. The values are an index, not IBM pricing, and the point is the shape, not the number.
The figures are illustrative, but the lesson is exact: the move from full capacity to sub-capacity removed the idle machine from the bill, the largest single structural saving for most estates, and the move to consumption changed the question from managing a monthly peak to negotiating a baseline. Whether TFP beats a well managed WLC environment depends entirely on the baseline and growth terms you sign.
List every product and the metric it bills under. The mix is the map: it tells you which products respond to peak shaping, which need a metric move, and which are governed by a baseline. No optimization starts without it.
The wrong lever on the wrong metric wastes the year.
A product still on full capacity PSLC bills the whole machine. Where it is eligible, moving it to a sub-capacity metric can remove the idle capacity from the bill in one step. That is the largest structural lever in the history.
Full capacity on a half used machine is pure overpayment.
Tailored Fit Pricing is not automatically cheaper. Model your actual consumption and growth against a well managed WLC bill before you commit. The baseline and growth clauses decide whether TFP wins, not the brochure.
Consumption pricing is only as good as its baseline.
Newer machine generations carry their own metric variants and price tables. A hardware refresh is the natural moment to revisit which metric each product sits on and to renegotiate the terms around it.
The refresh is the window; use it before it closes.
From full capacity to sub-capacity to consumption. PSLC billed the whole machine in the 1990s; WLC introduced sub-capacity on the R4HA in 2000; AWLC and variants refined it across generations; Tailored Fit Pricing moved to annual consumption in 2019.
Full capacity bills the total rated MSU of the machine. Sub-capacity bills the R4HA peak of the LPARs running the product, via SCRT. Since machines run above the everyday peak, the move to sub-capacity was the biggest structural saving for most estates.
IBM's 2019 consumption model. It bills actual annual MSU against a baseline rather than the monthly peak, aiming for a more predictable bill. Whether it beats managed WLC depends entirely on the baseline and growth terms negotiated.
Because estates run a mix of metrics and each has different levers. Peak shaping moves WLC but not PSLC; a TFP container is governed by its baseline. Mapping metric to product is the start of any optimization.
Related: licensing concepts hub · sub-capacity vs full capacity · zNALC explained · IBM licensing hub · MSU optimization
Audit notice or renewal under 18 months out? We mobilize within 48 hours.
Get expert help →