iCan Blog Archive

This is the first installment in a two-part blog series to provide basic introduction to the PowerVM processor virtualization terminology and acronyms. The first part will be heavy on terms without much explanation; the second part will be just the opposite. The IBM Redbooks publication, “IBM PowerVM Virtualization Introduction and Configuration” provides much greater detail. For an in-depth treatment of this material from an IBM i perspective, see “Under the Hood: POWER7 Logical Partitions.” Let’s get started…

The IBM i operating system executes in a system virtual machine (VM) implemented by the PowerVM hypervisor on a Power Systems server. PowerVM supports 1,000 concurrent instances of VMs, which are called logical partitions (LPARs). The hardware resources available to the LPAR are specified in the LPAR configuration, and include the number of virtual processor cores (VCPUs), MBs of mainstore, etc., as well as resource virtualization attributes and qualifiers. For instance, the LPAR may use shared processors, in which case the processing units defines the fractional share of physical processor dispatch time to which the partition is entitled, or the partition may use dedicated processors, in which case the processing units is implicitly the full share of physical processor dispatch time corresponding to the number of VCPUs configured. The term entitled capacity (EC) is sometimes used interchangeably with processing units, especially to avoid confusion with [virtual] processors.

There are a number of LPAR configuration parameters associated with processors. Some of them are fixed for the duration of the partition activation (i.e., partition IPL), and some may be changed while the partition is active. Static LPAR parameters include the minimum and maximum number of VCPUs, the processing mode (Shared or Dedicated), the processor sharing mode, and the Processor Compatibility Mode (PCM). A shared processor LPAR in uncapped sharing mode is often simply referred to as an uncapped partition, as its virtual processor dispatch time is not limited, or capped, by its entitled processor units. Dynamic LPAR parameters include the current number of VCPUs, the current processor units for a shared processor LPAR (SPLPAR), the weight for an uncapped partition, and the processor-sharing attribute for a dedicated processor LPAR. The term donation attribute is often used in place of processor sharing attribute to underscore that sharing is the result of a dedicated processor LPAR temporarily donating unused processors to the physical shared processor pool.

Many of the parameters for a dedicated processor LPAR are shown in Figure 1 below, which is the Processors properties tab of a partition profile as rendered by the HMC. Figure 2 illustrates the parameters for a shared processor LPAR.


Figure 1 – Dedicated Processor Sharing Mode Attributes


Figure 2 – Shared Processor Sharing Mode Attributes

Of all of these parameters, Processor Compatibility Mode is probably least familiar to IBM i audiences. The short explanation is that PCM makes the virtualized processor appear as an instance of the indicated generation of POWER processor. So for example, POWER8 mode supports SMT8 and implements Version 2.07 of the Power Instruction Set Architecture (ISA), POWER7 mode supports SMT4 and implements Power ISA 2.06, POWER6 mode supports SMT2 implements Power ISA 2.03, and so forth. On a POWER8 server, PCM can be configured for POWER8, POWER7, POWER6 and POWER6+.

One of the benefits of the Technology Independent Machine Interface (TIMI) is that IBM i applications are generally insulated from the Power ISA version. As a result, PCM has not been relevant for most IBM i users. Now that IBM i supports Live Partition Mobility between POWER7 and POWER8 Power Systems servers, PCM will become more important, because it must be set to a mode supported on both the source and target systems.

In the next part, we’ll look at how these partition configuration parameters interrelate, and the flexibility that PowerVM processor virtualization can offer in support of scale-up and consolidation roles, on the same platform, at the same time.

I’d like to thank Chris Francois for writing this blog. Chris is one of the leading experts on IBM i running on the POWER processor. Chris has been a commercial OS kernel programmer for nearly 25 years, and is currently a lead developer of IBM i Licensed Internal Code. Chris joined IBM Rochester in 1994 during the AS/400’s migration to the processor family ultimately to be known as POWER. In addition to authoring this blog, Chris is also the author of a recent article on IBM i developerWorks, IBM i 7.2 and POWER8.

This blog post was edited to fix broken links on April 11, 2020.

This blog post was originally published on IBMSystemsMag.com and is reproduced here by permission of IBM Systems Media.