On any meaningfully virtualized estate, sub-capacity is the cheaper counting basis — often by a multiple — but it is not a discount, it is a conditional licence: deploy an approved metering tool within 90 days, keep it healthy, generate quarterly reports and retain them for two years, or the right to count partition cores evaporates and an audit counts every core in the box. The real comparison is therefore not economic but operational: full capacity costs more and demands nothing; sub-capacity costs less and demands permanent discipline.
Published 6 November 2025 · Last reviewed 6 November 2025
The Processor Value Unit is IBM’s long-serving capacity metric for distributed software: every processor core is assigned a PVU weighting from IBM’s processor table according to the chip’s family and model, and the licence requirement is cores multiplied by that weighting. The metric itself is not the decision. The decision is which cores get counted, and IBM’s Passport Advantage terms offer exactly two answers.
Full capacity counts every activated core in the physical server — or, where products move freely across a cluster, in the cluster — regardless of how little of it the IBM software can actually use. A WebSphere instance pinned to four virtual cores on a 64-core host licenses all 64. It is the default basis, the one audits fall back to, and the one that requires no tooling, no reports and no eligibility tests.
Sub-capacity — IBM’s term is Virtualization Capacity — counts only the cores available to the partitions or virtual machines where the product runs, capped at the physical capacity of the host. The same WebSphere instance licenses four cores. On consolidated, virtualized infrastructure the difference between the two counts is routinely severalfold, which is why nearly every sizeable IBM customer licenses sub-capacity — and why the conditions attached to it are the single most consequential fine print in the IBM relationship.
This guide compares licensing programs as published by the vendor; it is general information, not legal or licensing advice for your situation, and it names no firms. Program terms change — verify current terms against your own Passport Advantage agreement. The firm directory lists IBM-capable advisors with balanced pros and cons, listed, not ranked.
| DIMENSION | FULL CAPACITY | SUB-CAPACITY (VIRTUALIZATION CAPACITY) |
|---|---|---|
| What is counted | Every activated core in the physical server or eligible cluster | Cores available to the partitions/VMs running the product, capped at physical capacity |
| Eligibility | None — the default basis, available everywhere | Product, virtualization technology and operating system must each be on IBM’s eligibility lists |
| Tooling | None required | ILMT, BigFix Inventory or an approved Flexera One configuration, deployed within 90 days of first eligible deployment and kept current |
| Reporting | None | Reports generated at least quarterly and retained for two years |
| Licence cost shape | Highest count the hardware allows; rises with every host refresh to bigger boxes | Tracks actual allocation; consolidation and right-sizing flow straight through to the count |
| Operational cost shape | Effectively zero | A permanently owned tool estate: agents, PVU-table updates, report sign-off, someone accountable |
| Audit posture | Simple to evidence; little to dispute | The audit’s first question is whether the conditions held; if not, the position reverts to full capacity retroactively |
| Where the risk sits | Overpayment by design — certain, budgeted, visible | Compliance failure by neglect — contingent, unbudgeted, discovered in audit |
Read the last row twice. Full capacity’s cost is a known overpayment you choose; sub-capacity’s cost is a contingent liability you manage. Most of the genuinely large IBM audit findings are not about deploying too much software — they are about sub-capacity conditions that quietly failed, converting years of partition-level counting into server-level counting in a single finding. That conversion, not the licence ledger, is what an IBM audit defense engagement most often ends up contesting.
Sub-capacity fits any estate where IBM products run on shared, virtualized infrastructure at meaningful scale — which is to say, nearly all of them. The multiple between partition cores and physical cores is the entire business case, and it usually dwarfs the cost of operating the metering estate properly. The qualifier is organisational: sub-capacity fits estates where someone owns ILMT as production infrastructure, not as a checkbox — with agent coverage tracked, the PVU table updated, and quarterly reports actually generated and reviewed.
Full capacity fits three defined situations. Bare-metal deployments, where the two counts are identical and the tooling buys nothing. Small, static footprints where the licence delta is smaller than the standing cost of compliance machinery. And environments that fail the eligibility lists anyway, where full capacity is not a choice but the only available basis. IBM’s terms have also carved narrow exceptions allowing manual tracking for the smallest enterprises and lowest-capacity environments — still with quarterly worksheets, and with thresholds you should verify against the current Passport Advantage terms rather than assume.
The test that decides it: take your largest virtualization cluster and compute both counts for the IBM products on it. If the ratio is well above one — it usually is — sub-capacity is the only defensible basis, and the budget conversation should move immediately to funding the discipline it requires. If the ratio hovers near one, you are paying for compliance machinery that saves nothing, and simplification is on the table.
ILMT health is the audit. In a PVU audit, the deployment data is largely settled by the tool; the contest is over whether the sub-capacity conditions held. Missing agents, an outdated PVU table, unsupported tool versions, gaps in the quarterly report record — each is an argument for counting some slice of the estate at full capacity. A standing IBM compliance assessment that treats ILMT output as evidence to be audited, not gospel to be filed, finds these failures while they are correctable.
The 90-day clock starts at deployment, not at discovery. Eligibility requires the metering tool within 90 days of the first eligible sub-capacity deployment. Estates that virtualized first and tooled up later carry a permanently unqualified period — and audit findings concentrate precisely there, because the terms offer no retroactive cure. If that history exists in your estate, quantify the exposure before the vendor does and bring it, framed, into the next commercial event.
Findings are renewal currency — for both sides. IBM audit outcomes habitually resolve into commercial settlements: the finding becomes a Cloud Pak adoption, an ELA, a renewal uplift. Knowing your true position — including exactly how defensible your sub-capacity record is — decides whether you negotiate that settlement from evidence or from fear. The same record is leverage in the other direction: a clean, continuous report history removes the auditor’s easiest multiplier and shortens the engagement.
The metric is migrating around you. IBM’s catalogue has been shifting from standalone PVU licences toward Cloud Paks and Virtual Processor Core metrics, where containerised deployments are metered by the IBM License Service rather than ILMT. Conversion offers change the counting basis, the tooling and the audit surface all at once — evaluating one is a licensing-advisory exercise, not a procurement formality, and it is exactly the work an IBM licensing advisory engagement structures.
Treating ILMT as installed-equals-compliant. The tool existing is not the condition; the conditions are coverage, currency and reports. The classic failure is an ILMT server faithfully running for years while a quarter of the estate’s agents went silent — every uncovered deployment is a full-capacity argument waiting for an auditor.
Letting infrastructure teams move workloads the licence didn’t follow. Live migration across hosts and clusters is routine operations — and every move changes the eligible core count. Cluster expansions are the quiet killer: adding hosts to a cluster where IBM products float raises the cap even if nothing was redeployed. Licensing review belongs inside the change process, not after it.
Assuming every product and platform qualifies. Sub-capacity eligibility is a three-way test — product, virtualization technology, operating system — against lists IBM maintains and updates. Estates that standardise on a platform before checking the lists occasionally discover whole clusters that were never eligible, and have been mis-counted since day one.
Decommissioning the evidence. Reports must be retained for two years, and audit lookbacks reach further. Tool migrations, server rebuilds and tidy-minded cleanups that discard old ILMT databases delete the only proof that past quarters were compliant. Archive before touching anything.
Taking remediation advice from the party that profits by it. Post-finding, offers arrive to make it all go away inside a bigger commitment — from the vendor, and sometimes from advisors whose economics ride on the deal. The independence test applies with full force here: ask who profits from the settlement being shaped the way it is.
No. IBM’s terms accept the IBM License Metric Tool, IBM BigFix Inventory and approved Flexera One configurations with IBM IT asset management, deployed within 90 days of the first eligible deployment. ILMT is the most common because it is available at no charge — but whichever tool is used must be kept current, cover every eligible deployment, and produce the required quarterly reports.
The estate does not qualify for sub-capacity counting, and in an audit every PVU deployment is counted at the full physical capacity of the server or cluster it runs on — on a heavily virtualized estate, severalfold the partition-level count. Installing ILMT late stops the exposure growing but does not retroactively qualify the period before deployment, which is typically where the finding concentrates.
Narrow ones. IBM’s terms have allowed manual tracking where the enterprise is small enough or the eligible environment’s total capacity is low enough, and where the tool does not support the virtualization technology in use. The exceptions still require manual quarterly reporting on IBM’s worksheets — verify the thresholds against the current Passport Advantage terms rather than assuming an exception applies.
No. The product, the virtualization technology and the operating system must each be on IBM’s eligibility lists, reports must be generated at least quarterly, and the reports retained for two years. A running ILMT instance with missing agents, an outdated PVU table or an incomplete report record fails the conditions in audit practice even though the software is technically installed.
Yes, in defined cases: bare-metal deployments where the two counts are identical, small static estates where the licence delta is less than the cost of operating the compliance machinery, and environments that fail eligibility anyway. The mistake is reaching full capacity by neglect rather than by decision.
No. This is a directory, not a ranking. Firms are listed alphabetically with balanced pros and cons. Independence is shown as a pro and reseller, Big-Four or vendor-side-audit ties as a con, both stated as factual trade-offs for you to weigh.
Computing both counts across your clusters, auditing the health of the sub-capacity record before IBM does, and turning that record into leverage at the next commercial event — that is precisely the work of an IBM licensing advisory engagement. The IBM hub maps the vendor’s wider licensing world, and the directory lists every firm covering IBM — with balanced pros and cons, listed, not ranked. Choosing between them? Start with how to choose an IBM licensing partner.
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