① Journal · Cost control
Sub-capacity pricing lets you pay on the peak you consume, not the box you installed. But the entitlement only pays if the rolling four hour average is held and the report is filed clean every month. The discipline costs almost nothing. Letting it slip costs the entire gap to full capacity.
The entitlement is free. The discipline is what cashes it.
Sub-capacity pricing is the single most valuable cost mechanism on the IBM mainframe and the one most often left half used. The principle is simple: instead of paying for every installed MSU, you pay on the rolling four hour average peak your workload actually drives. The savings against full installed capacity are large, because almost no estate runs flat out for four straight hours. But the entitlement is conditional. Two things have to stay true every month, and both are easy to let drift.
The first is the report. The Sub-Capacity Reporting Tool measurement has to be run, validated, and submitted on the vendor's schedule, every period, without gaps. Where a report is late, incomplete, or inconsistent, the vendor is entitled to treat the period as noncompliant and bill it at full installed capacity, which erases the entire benefit for that month. The second is the peak. Sub-capacity rewards a managed rolling four hour average and quietly penalizes an unmanaged one, because the charge keys off the single highest peak in the month. Hold the peak and file clean, and the saving recurs. Let either slip and it evaporates. Read this with our sub-capacity vs full capacity explainer and our MSU optimization service.
Illustrative monthly profile · the bill follows the single highest R4HA, not the average
| Scenario | R4HA that bills | Charged as | Effect |
|---|---|---|---|
| Full capacity, no sub-capacity | Not applicable | Total installed MSU | Pay for every MSU on the box, used or not |
| Sub-capacity, peak unmanaged | Highest unmanaged peak | The full monthly peak | One spike sets the bill for the whole month |
| Sub-capacity, batch rescheduled off peak | Lower peak after the move | The reduced peak | Same work, lower billed peak, recurring saving |
| Sub-capacity, soft capped | The cap, where work is deferrable | The cap value | Peak held at the chosen MSU level on purpose |
| Sub-capacity, report missed | Disregarded | Full installed capacity | Reverts to full capacity, the benefit is lost |
Illustrative, not a quote. Exact reversion behavior and reporting cadence follow the published sub-capacity terms and the agreement; the pattern, that the peak and the filing decide the charge, is what matters here.
Treat the SCRT submission as a not negotiable monthly close, owned by a named person with a backup. A missed or inconsistent filing is the one mistake that reverts the whole period to full installed capacity. The cheapest saving on the frame is the one you simply do not forfeit.
Never let a filing slip; the gap is the bill.
The bill follows the highest rolling four hour average, so the only hours that matter are the peak ones. Measure them, attribute them to workloads, and treat that peak as a managed number rather than a byproduct. Capacity you cannot see in the peak is capacity you cannot hold down.
Manage the peak hour, not the daily average.
Where the peak is driven by deferrable work, defined and group capacity limits hold the rolling four hour average at a chosen MSU level without starving the online day. Soft capping is a deliberate buyer side cost control, aimed at the costly window, not a blanket throttle on the system.
Cap the peak the meter reads, nothing else.
Deferrable batch that runs into the online peak stacks two demands inside the same four hour average. Move it out and the peak falls with no real work removed. The cheapest MSU is the one you never place on the peak, and rescheduling is the lowest cost lever of them all.
Spread the demand, lower the billed peak.
④ Why we call it insurance
Sub-capacity is a discount you have to keep earning. The monthly discipline is what keeps it. Cheap to hold, expensive to drop.
Typical reduction negotiated on renewal spend
Mainframe spend negotiated on the buyer side
Engagements delivered since 2019
The routine of managing the rolling four hour average peak that drives software charges and filing the SCRT report accurately and on time every month. Sub-capacity lets you pay on consumed peak rather than installed capacity, but only if the reporting is clean and the peak is actively held. The discipline is the monthly habit that turns the entitlement into a real saving.
It is one of the fastest ways to lose the benefit. Where reporting is late, incomplete, or inconsistent, a vendor can treat the period as noncompliant and revert the charge to full installed capacity, which on a large box is far more than the consumed peak. The timing is not negotiable, and the cost of a slip is the entire gap to full capacity for that period.
The charge follows the highest rolling four hour average in the month, so the bill is set by a handful of peak hours, not the average load. Soft capping holds the peak where it pays to, and rescheduling deferrable batch lowers it without removing real work. Because the charge keys off that single peak, a small reduction in the right hours recurs every month. See our soft capping wins.
The R4HA and SCRT mechanics are IBM specific, but the principle that consumption discipline protects a capacity based charge applies across publishers whose renewals key off measured MSU or MIPS. Wherever a metered peak sets the bill, the same habits, hold the peak and report it cleanly, protect the buyer. Our cost optimization service applies them across the estate.
Related: the hidden cost of MIPS creep · soft capping wins · MSU optimization quick wins · MSU optimization service · cost optimization
Audit notice or renewal under 18 months out? We mobilize within 48 hours.
Get expert help →