MartinPacker 11000094DH Visits (4508)
So now I get to sit and relax for umpteen hours, while a 787 speeds me to California. So I can write something. And, yes, it’s been a while.
To tell you the truth, I’ve been working on back-to-back engagements and enjoying vacation for what seems like forever. And after a stint of that I generally I end up with a swirl of “if only I had time to write about that” topics in my brain. So here I am ”Going To California On A Big Jet Plane” with time on my hands.
So I have this customer - and it’s not the first like it - who has a Parallel Sysplex with a pair of Coupling Facility LPARs and a couple of Production z/OS LPARs. Not the first, either, to have offline Coupling Facility links. But I didn’t know they had offline links until they told me. Shame on me!
If a customer tells me things I guess I should know I go on a hunt for how I could have figured it out from the data. And so it was in this case. And, having just been successful at writing code to detect offline links, I’m writing this to show you what’s possible and how.
Three Sources Of Link Data
SMF 74 Subtype 4 is the main Coupling Facility Activity record, and it has three sets of information about links:
You get at least one 74–4 for each coupling facility from each z/OS connected to it. Ideally you’d enable 74–4 on all systems in the sysplex.
I say “at least” as modern CFs tend to spill to a second and, occasionally, a third. So here’s another thing I’ve absorbed over the last few customer engagements: Customers nowadays have hundreds of structures in each coupling facility.
Local Facility Data Section
You get one of these in the 74–4 record - whether the base or the overflow ones. It has to be there as it has the Coupling Facility name, as well as the machine it’s on.
It also has some relevant things which I’ll touch on now, and come back to later:
As examples of Channel Path Acronyms, I’ll take three:
Remote Facility Data Section
You get one or more of these, depending on how many CFs the one in the Local Facility Data Section has duplexing links to. The path-related information in this section is:
Pretty similar to Local Facility Data Section information. But not quite. The information is only for online paths.
Path Data Section
I’ve written about this section many times, most recently in And Latency Once More.
You get one section for each link under certain circumstances: The link has to be physical (no IC links) and modern enough (not ICB, for example). Unfortunately, it has to be online. So this section doesn’t help us in our quest for offline links.
And the reason I keep writing about this section is because it is full of value and surprises. But not this time.
Actually There’s A Fourth
I include this for completeness. SMF 73 gives information on CHPIDs - from a z/OS system.
For CF links this information is very basic. Mainly the CHPID, whether it’s shared, whether it’s online, and Channel Path Acronym.  For other types of channel, such as FICON, we get a lot more, useful, information.
There is a potential use for this data, quite apart from corroboration: If a CF-related channel path appears in SMF 73 but not in 74–4 it implies it’s not connected. Of course it would be strange if it managed to be online despite that. I should probably code to this possibility.
The Truth Is Masked
In the Local Facility Data Section piece I mentioned path masks. It might not have seemed obvious but they are key to this. There are three 8–bit mask fields:
Each bit represents a different path and each path is described by the same bit position in all the masks.
These terms might be familiar in the context of disk controllers.
Consider the following worked example:
This is from a real customer. Reading from top to bottom.
Taken together, we conclude:
For links between z/OS LPARs and CF LPARs we can see both the online and offline paths. For the CF to CF links we can only see the online ones.
There is an old joke. It goes like this, updated a little: “I wrote a long blog post as I didn’t have time to write a short one”. Well, I did have time to write a short one…
By the way “Maskerade” is the name of a Terry Pratchett Discworld book, in case you wondered if there was a cultural reference. High time I read it again.