Partition sizing recommendations for IBM i 7.5
As is typical with a new release, the IBM® i Operating System has made changes to the underlying infrastructure that may require more resources in the configuration. In some cases when upgrading from an earlier release, infrastructure changes may result in a need to increase the amount of resources to continue to run the same workload. As with any new release, the CPU or I/O signature of any given workload may change.
For an example relating to memory, a very minimal configuration contains just the base operating system with a console. This minimum is proof that you can simply install and IPL the base operating system. The amount of disk in this configuration is also minimal -- only a protected load source disk. This minimal configuration requires at least 4 GB of partition memory.
Generally, with the need to run useful workloads in production, most realistic small configurations will require additional memory. For example, virtual disk needs more memory than dedicated disk, and communications devices also require larger amounts of memory. If upgrading from an earlier release that is already running with more than this minimum amount of memory, infrastructure changes may still result in a need to increase the amount of memory to continue to run.
A memory-constrained partition may not IPL in the worst case. Rather, it will hang in an early IPL step as the OS is unable to allocate the data structures it requires. Lesser constrained partitions will see excessive paging as work loads start to run. Adding additional memory will alleviate the problem.
- Performance FAQ, sections such as Sizing a System, and Memory Questions
- IBM Documentation topic Identifying and resolving common performance problems