Clarify and Enhance zIIP Boost Behavior Under System Recovery Boost

The System Recovery Boost (SRB) feature, introduced with IBM z15, is designed to provide additional CPU capacity during recovery operations, such as IPLs or middleware restarts. One of the core capabilities of SRB is the zIIP Boost, which is described in IBM documentation as enabling z/OS to “make normally nonzIIP eligible work zIIP eligible” by “blurring the CPs and zIIPs together”

(Source: https://publibz.boulder.ibm.com/zoslib/pdf/z15SystemRecoveryBoostWhitePaper.pdf)

These statements have led many customers to believe that, during the recovery periods, “all types of workloads” may execute on zIIP engines without traditional eligibility constraints.

However, based on our testing of the real recovery scenarios, we observed that only a limited subset of non-zIIP-eligible work is actually redirected to zIIPs. Unfortunately, some of the tasks excluded from zIIP Boost are critical for accelerating recovery.

To better align the feature behavior with its intended purpose and customer expectations, we propose two actions:

1-     Functional Enhancement:

Where technically feasible, allow all z/OS tasks -regardless of their standard zIIP eligibility- to be executed on any available GCP or zIIP engine during an SRB window. This would deliver on the promise of “blurred” execution and maximize performance during critical recovery moments.

2-     Documentation Clarification:

If there are architectural or technical limitations that prevent such universal eligibility, we kindly request a clear update in IBM’s official documentation. This clarification should  explicitly state that only certain types of non-zIIP-eligible workload benefit from zIIP Boost under SRB, thereby setting accurate expectations.

Providing either a behavioral enhancement or improved documentation would ensure customers can plan their recovery procedures and capacity with greater confidence and transparency.