Coupling Facility Subchannel Busy in RMF

(Originally posted 2008-07-11.)

In the same meeting with RMF Development as ERBSCAN / ERBSHOW was discussed we talked about Parallel Sysplex instrumentation. (Both XCF and Coupling Facility aspects.)

One thing I hadn’t noticed is the appearance of Subchannel Busy Percentage as a number in the following three places:

  • In Monitor III
  • In the Spreadsheet Reporter
  • As Overview Condition (“SUBCHBP”)

I think you can do useful work with this but you have to know a few things about it:

  • It is one system’s view.
  • It is an estimate, derived from request rates and service times, both Sync and Async. Essentially you work out the request total time in the RMF interval and divide that by the interval length and the number of subchannels. The implicit assumption is that the service time is all subchannel busy time and that there is no other time when the subchannel is busy. It’s a moderately well-known formula, however.
  • It says nothing about individual subchannel busy levels – nor about paths.

This topic is of interest to me for a couple of reasons:

  • On our Residency in May / June we used coupling facilities that were attached by a mixture of ICB and ISC links. Two different link types between the same z/OS and the same coupling facility. It would’ve been really nice to see which link type actually got favoured.
  • My suspicion is that customers would like to do more detailed connectivity analysis and tuning for their parallel sysplexes. For example, if a link doesn’t get used it’d be nice to know that and to be able to trouble-shoot it. (Perhaps someone left it connected as a 20km fibre suitcase.) 🙂

So, I’m wondering… Is link and connectivity information important? What do you need? And why? Or is the Subchannel Busy Percentage estimate good enough for you to work with?

Oh, and thanks to Matthias Gubitz for the phrase “Gutes tun und daruber sprechen” (“Do good and speak about it.”) 🙂 That’s probably the best reason for blogging and such like.

And the “neologism of the week” award goes to Harald Bender for the word “handisch”. Neither English nor German. 🙂 We believe he meant “by hand”. But for now I prefer “handisch”. 🙂

Published by Martin Packer

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