My Generation?

Ten years ago in Channel Performance Reporting I talked about how our channel-related reporting had evolved.

That was back when the data model was simple: You could tell what channel types were and what attached to each channel:

  • Field SMF73ACR in SMF Type 73 (RMF Channel Path Activity) told you the channel was, for example, FICON Switched (“FC_S”). This was an example I gave in that blog post.
  • SMF Type 78 Subtype 3 (RMF I/O Queuing Activity) gave you the control units attached to each channel. (You had to aggregate them through SMF 74 Subtype 1 (RMF Device Activity) to relate them to real physical controlers.)

SMF 74 Subtype 7 FICON Switch Statistics

In that post I talked about SMF 74 Subtype 7 (FICON Director Statistics) as a possible extension to our analysis. I mapped this record type pretty comprehensively. Unfortunately so few customers have 74–7 enabled that I haven’t got round to writing analysis code.

I’m grateful to Steve Guendert for information on how to turn on these records:

RMF 74–7 records are collected for each RMF interval if

  • The FICON Management Server (FMS) license is installed on the switching device. (This is the license for FICON CUP)
  • The FMS indicator has been enabled on the switch
  • The IOCP has been updated to include a CNTLUNIT macro for the FICON switch (2032 is the CU type)
  • The IECIOSnn parmlib member is updated to include “FICON STATS=YES”
  • “FCD” is specified in the ERBRMFnn parmlib member

With these instructions I’m hoping more customers will turn on 74–7 records.

Talking About My Generation

What I don’t think we had 10 years ago was a significant piece of information we have today: Channel Generation. This is field SMF73GEN in SMF Type 73.

This gives further detail on how the channel is operating.

There isn’t a complete table anywhere for the various codes but here’s my attempt at one:

Value Meaning Value Meaning
0 Unknown 18 Express8S at 4 Gbps
1 Express2 at 1 Gbps 19 Express8S at 8 Gbps
2 Express2 at 2 Gbps 21 Express16S at 4 Gbps
3 Express4 at 1 Gbps 22 Express16S at 8 Gbps
4 Express4 at 2 Gbps 23 Express16S at 16 Gbps
5 Express4 at 4 Gbps 24 Express16S at 16 Gbps
7 Express8 at 2 Gbps 31 Express16S+ at 4 Gbps
8 Express8 at 4 Gbps 32 Express16S+ at 8 Gbps
9 Express8 at 8 Gbps 33 Express16S+ at 16 Gbps
17 Express8S at 2 Gbps 34 Express16S+FEC at 16 Gbps

Actually that “24” entry worries me.

By the way I’m grateful to Patty Driever for fishing many of these out for me.

A Case In Point

We’ve been decoding these channel “generations” for a number of years but it came into sharper focus with a recent customer situation.

This customer has a pair of z14 processors, each with quite a few LPARs. A few weeks ago I pointed out to them that quite a few of their channels were showing up at 4 Gbps or 8 Gbps, not the 16 Gbps they might have expected.

Where the channels were at 16 Gbps they were indeed using Forward Error Correction (FEC).

Now, why would a 16 Gbps channel run at 8 or even 4 Gbps? It’s a matter of negotiation. If the device or the switch can’t cope with 16 the result might well be negotiation down to a slower speed. So older disks or older switches might well lead to this situation. I guess it’s also possible that unreliable fabric might lead to the same situation.

(Quite a few years ago I had a customer situation where their cross-town fabric was being run above its rated speed, with quite a lot of errors. It was a critsit, of course. 🙂 Later I had another critsit where the customer had ignored their disk vendor (not IBM) ’s advice in how to configure cross-town fabric. So it does happen.)

Apart from SMF73ACR the above all relates exclusively to FICON. So what about Coupling Facility links?

I’ve written extensively on these. The most recent post is Sysplexes Sharing Links. In this post you’ll find links (if you’ll pardon the pun) to all my other posts on the topic. I don’t need to repeat myself here.

But SMF73ACR does indeed identify broad categories of Coupling Facility links:

  • “ICP” means “IC Peer link”.
  • “CS5” means “ICA-SR link”.
  • “CL5” means “Coupling Express LR link”.

Conclusion

Whether we’re talking about Coupling Facility links or FICON links it’s worth checking what you’re actually getting. And RMF has – both in its reports (which I haven’t touched on) or in its SMF records – the information to tell you how the links panned out.

It’s also worthwhile knowing that raw link speed doesn’t translate directly into I/O response time savings. But modern channels, especially when exploiting zHPF, can yield impressive gains. But this is not the place to go into that.

One final thought: Ideally you’d allow the latest and greatest links to run at their full speed. But economics – such as the need to keep older disks in Production – might defeat this. But, over time, it’s worthwhile to think about whether to upgrade – to get the full channel speed the processor is capable of.

Published by Martin Packer

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