What is IBM Power 8 Systems compatibility mode?

Solution In Progress - Updated -

Environment

  • IBM Power Systems server based on POWER 8 processor
  • Operating system based on the Linux kernel

Issue

  • What is IBM Power 8 Systems compatibility mode?

Resolution

  • 'Compatibility mode' simply means that the POWER8 processor will act
    and appear in a mode consistent with an earlier generation processor,
    for example, a POWER7.

  • When running Linux on a POWER 8 system, users will sometimes see
    that the system is running in a `compatibility mode'. Whether the
    system runs in native mode or compatibility mode is determined
    by which kernel is running on the system. On boot, the Linux kernel
    provides the hypervisor with information on what capabilities it
    supports (which includes the list of CPU versions it supports).
    They hypervisor has a list of allowed compatibility modes, which
    on POWER8 would normally be P8, P7 and P6. The hypervisor then
    chooses the highest mode that is in both lists.

  • As applied to Red Hat Enterprise Linux, in practical terms this means
    the following:

    • POWER 8 systems are fully supported in native mode starting
      with RHEL 7.0 and subsequent updates only
    • POWER 8 systems are fully supported in P7 compatibility mode
      starting with RHEL 6.5 and subsequent updates and RHEL 7.0 and
      subsequent updates.

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