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About Processor Modes

May 09, 2017

During the build out of a new POWER8 server, we were loading VIO server. We were using the classic HMC interface, and a user clicked on the profile after VIOS was running. He checked the hardware tab, and in that tab, they happened to see the processor compatibility mode.

What would you expect to see there?

Naturally, you'd expect to see POWER8 mode. But instead, POWER7 was the compatibility mode being displayed.

An AIX LPAR did show POWER8 mode. So why wasn't POWER8 appearing in the VIO server LPAR? For that matter, why are there differences in processor modes anyway?

Here's a good overview of what I'm talking about. But the short version is this: There are POWER6, POWER7 and POWER8 processor modes. One advantage with these nodes is that they can be used to enable live partition mobility operations between different server families. That helps when migrating from POWER6 or POWER7 to POWER8 servers, because it allows you to take an outage to change the processor mode at your convenience. In effect, you can migrate without any downtime.

There's also a default processor mode. From the same link:

The default processor compatibility mode is a preferred processor compatibility mode that enables the hypervisor to determine the current mode for the logical partition. When the preferred mode is set to default, the hypervisor sets the current mode to the most fully featured mode supported by the operating environment. In most cases, this is the processor type of the server on which the logical partition is activated. For example, assume that the preferred mode is set to default and the logical partition is running on a POWER8 processor-based server. Because the operating environment supports the POWER8 processor capabilities, the hypervisor sets the current processor compatibility mode to POWER8.

But back to the original issue: Why was VIOS coming up in POWER7 mode? VIO server is based on AIX 6.1. The hypervisor has determined that POWER7 compatibility mode is the best mode in which to run. This is also confirmed in this article:

Once the VIO server was running, I went through all the normal checks. Ioslevel shows as 2.2.3.3 and oslevel –s shows the operating system at 6100-09-03-1415. This means the VIO will be running in SMT4 mode since SMT8 requires 7.1 tl03 sp3.

So it's nothing to worry about; it's just another thing to be aware of as you move to POWER8.

Posted May 09, 2017 | Permalink

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