Processor compatibility mode
The setting on each partition that decides which generation of Power processor features the operating system inside it is allowed to use. For an IBM i shop it is the reason a Linux AI partition on a new Power 11 server can still be running as a Power10 or POWER9 machine, which changes what the inference software underneath can reach.
What does processor compatibility mode do?
A Power server is cut into logical partitions, or LPARs. Each partition carries a processor compatibility mode. That mode tells the PowerVM hypervisor which generation of Power processor features the operating system inside the partition is allowed to use.
Think of a rental car with a speed limiter. The engine can do more, but the limiter decides what the driver gets. A Power 11 core left in Power10 mode behaves like the older generation on purpose, so older software keeps working.
IBM's Power 11 Redbook says the mode is defined at the partition level when the partition is created, and that partitions on the same physical server can run in different modes at the same time. So one box can hold an IBM i partition in one mode and a Linux AI partition in another.
There are two numbers to read, not one. The configured mode is what you typed. The effective mode is what the partition got. IBM says the hypervisor checks the configured mode at activation, and if the operating system cannot support it, the hypervisor drops down to the most fully featured mode that system does support. It can go down. It can never go up past what you configured. You cannot change the effective mode while the partition is running.
Which modes does a Power 11 server offer?
The modes IBM documents for a Power 11 server
| Mode | What the operating system may use | Servers it can run on |
|---|---|---|
| POWER9_Base | POWER9 features, as enabled by firmware FW910 or later | Power11, Power10, POWER9 |
| POWER9 | POWER9 features plus enhanced interrupt and compression co-processor support, firmware FW940 or later | Power11, Power10, POWER9 |
| Power10 | Power10 features | Power11, Power10 |
| Power11 | Power11 features | Power11 only |
| Default | The hypervisor picks the most fully featured mode the operating system supports | Depends on the mode it picks |
Source: IBM Documentation, Processor compatibility mode definitions, IBM Power E1180 (9080-HEU). The same page for Power10 servers also lists a POWER8 mode and names firmware FW1010 for Power10 mode. A Power 11 server does not list POWER8.
The mode also caps how big one partition can get. IBM's support note on partition limits, last updated 15 May 2026, says an IBM i partition in Power11 mode tops out at 48 SMT8 processor cores as standard, and can be taken to 240 SMT8 cores through an IBM Lab Services arrangement. POWER9 mode has the same 48 core standard limit but a lower expanded ceiling of 192.
Which mode will my AI software actually get?
This is the part that surprises people. The mode follows the level of the operating system or container platform, not the age of the server. A brand new Power 11 server can be full of partitions quietly running as Power10.
Operating system level and the mode it runs in on a Power 11 scale-out server
| Level | Mode IBM documents |
|---|---|
| AIX 7.3 TL 3 SP 1 or higher | Native Power11 |
| AIX earlier than 7.3 TL 3 SP 1 | POWER9 or Power10 |
| AIX 7.2 | POWER9 or Power10 |
| Red Hat Enterprise Linux 9.6 for Power LE or later | Native Power11 |
| Red Hat OpenShift Container Platform 4.18 | Power10 compatibility |
| Red Hat OpenShift Container Platform 4.19 or later | Native Power11 |
| Virtual I/O Server 4.1.1.10 | Power11 |
| Virtual I/O Server 4.1.0.40 | Power10 compatibility |
| Virtual I/O Server 3.1.4.60 | POWER9 compatibility |
Source: IBM Power11 Scale-Out Servers: Introduction and Overview, IBM Redbook SG24-8590, first edition October 2025. Red Hat's own OpenShift Container Platform 4.19 release notes list support for IBM Power11.
Read that table against your AI plan. Red Hat OpenShift AI runs on top of OpenShift Container Platform, so a cluster still sitting on 4.18 is running in Power10 compatibility mode on Power 11 hardware. The Linux partition that drives a Spyre Accelerator card needs Red Hat Enterprise Linux 9.6 or later, and that level is already high enough for native Power11 mode, so there is no reason to leave it pinned to an older setting.
Can an older mode slow down AI work?
IBM does not publish a feature by feature table of what each mode hides. What IBM does say is plain enough. The Power 11 Redbook warns that an older mode might restrict access to newer processor features and performance enhancements, and tells shops to treat it as a temporary step while they upgrade and recompile.
- Matrix Math Acceleration, the on-chip AI math IBM also calls Matrix-Multiply Assist, arrived with Power ISA v3.1 in Power10. It is not a POWER9 feature.
- IBM's Power10 performance quick start guide, November 2021, says code built with the compiler flag for Power10 will not run on POWER9 or earlier processors at all.
- That same guide gives a worked example of a mode gate on an accelerator. To use NX GZIP acceleration on a Power10 system, the partition must be in POWER9 mode, not POWER9_Base, or in Power10 mode.
- The same guide says a partition needs a power cycle when its compatibility mode is changed.
Treat this as a question to ask IBM, not a settled number. The documented facts above show that an accelerator feature can depend on the mode, and that Matrix Math Acceleration is newer than POWER9. They do not amount to an IBM statement of how much throughput a given model loses in a given mode. Before you size an inference partition, confirm the mode it will run in and ask IBM what that mode exposes.
What does it change when you move a partition?
Three checks before a Live Partition Mobility move
- 1. Moving a running partitionIBM says both the effective mode and the configured mode must be supported by the destination server. A partition running in native Power11 mode has nowhere older to land, because only a Power11 server supports that mode.
- 2. Moving a partition that is shut downOnly the configured mode has to be supported by the destination. If the configured mode is set to default, IBM says an inactive partition can move to any of the server types.
- 3. Changing the mode afterwardsThe effective mode cannot be changed while the partition runs. IBM's Power10 quick start guide says the partition needs a power cycle when the compatibility mode changes, so plan an outage window.
Sources: IBM Documentation, Effective and configured processor compatibility modes, IBM Power E1180 (9080-HEU), and IBM Power10 Performance Quick Start Guides, November 2021.
This matters for AI work because an inference partition is often the newest thing on the floor. If you want to be able to move it onto older Power hardware when the AI server is busy or down, the mode it was activated in decides whether that move is even allowed.
Related Terms
Live Partition Mobility
Live Partition Mobility moves a running IBM i, AIX or Linux partition from one IBM Power server to another without shutting it down, so an IBM i shop can empty a server for maintenance without an outage window. It is part of PowerVM Enterprise Edition, and IBM requires every I/O device on the moving partition to be virtual and served by a Virtual I/O Server.
AI Inference Partition
A separate supported operating-system partition used to execute an AI model while an IBM i partition may remain the production application and data source.
PowerVM
IBM's hypervisor for Power servers, built into system firmware. It is what lets a Linux partition running an AI model sit on the same machine as the production IBM i partition, each with its own processors, memory and cards.
Matrix Math Acceleration (MMA)
An on-chip acceleration feature in IBM Power10 and Power11 processors that speeds up the matrix math operations used in AI inferencing, without requiring a separate GPU.
Virtual I/O Server
VIOS is a dedicated IBM Power partition that shares one set of physical disk, Fibre Channel and Ethernet adapters out to every other partition on the server. For an IBM i shop that means a Linux partition running an AI model can get its storage and its network connection from VIOS instead of buying its own physical cards.