The ILMT 90 day rule.

Sub-capacity licensing lets eligible customers license IBM software for the virtual capacity allocated to it rather than the full physical capacity of the host. That is a meaningful concession, and it comes with conditions. The central one is evidence. To rely on sub-capacity, an organization generally needs the IBM License Metric Tool (ILMT) installed and configured, with reports generated and retained.

The common requirement is to produce ILMT reports at least quarterly and to keep them for at least two years. A quarter is approximately 90 days, which is why practitioners talk about a 90 day rhythm. The point is not the exact day count. It is that the measurement of your environment must be continuous and current, not reconstructed after the fact when an auditor asks for it.

Three things have to be true on a rolling basis. ILMT has to see the environment, meaning every host that runs eligible sub-capacity software is discovered and monitored. The reports have to be generated on schedule, so that no quarter passes without a snapshot of consumption. And the history has to be retained, so the trail stretches back across the full look back period an audit can reach.

Each of these can fail quietly. A new hypervisor cluster comes online and never gets an agent. A version upgrade breaks collection and the gap goes unnoticed for two quarters. A server migration loses the older reports. Individually these look minor. Against the sub-capacity conditions, any one of them can undermine the eligibility of the affected environment.

If the evidence does not support sub-capacity for a given environment and period, IBM may apply full capacity licensing. That means counting the full physical core capacity of the host rather than the virtual cores allocated to the software. On a large virtualized estate, the gap between those two numbers is often several multiples. A product that is genuinely entitled in virtual terms can generate a substantial finding purely because the reporting trail has a hole in it.

This is what makes the 90 day rhythm a commercial issue and not just an operational one. The discipline of generating and keeping reports on schedule is, in effect, the thing that preserves the discount sub-capacity represents.

Treat ILMT as a controlled system, not a background utility. Confirm quarterly that coverage matches the current estate, that reports actually ran, and that the archive is complete and accessible. When infrastructure changes, check ILMT scope as part of the change, not afterward. And keep the report history organized so that, if a notice arrives, the evidence can be produced quickly and completely rather than assembled under pressure.

Where the history already has gaps, the priority shifts to remediation and to framing what the available data does support. A clean forward position paired with an honest account of the record changes the negotiation, even mid audit.

• Sub-capacity eligibility generally depends on ILMT, with reports produced at least quarterly and retained for at least two years.
• The 90 day rhythm is about continuous, current measurement, not a single deadline.
• A missed quarter for an environment can trigger full capacity licensing on the full physical core count.
• Coverage, schedule, and retention are three separate failure points, and any one can undermine eligibility.