Coupling Facility Structure Versions

When I see an 8-byte field in a record I think of three possibilities, but I’m prepared to discover the field in question is none of them. The three prime possibilities are:

1. A character field
2. A 64-bit counter
3. A STCK value

An interesting case occurs in SMF 74 Subtype 4: Two similar fields – R744SVER and R744QVER – are described as structure versions.

Their values are structure-specific. Their description is terse (as is often the case). By the way that’s not much of a criticism; One would need to write War And Peace to properly describe a record. I guess I’m doing that, one blog post at a time. 🙂

Some Detective Work

With such a field the first thing you do is get the hex(adecimal) representations of some sample contents. In my case using REXX’s c2x function. Here’s an example of R744SVER: D95FCC96 70EEB410.

A Character Field?

While not foolproof, it would be hard to mistake an EBCDIC string’s hex values for anything else. And vice versa. (Likewise ASCII, as it happens.) I think you’ll agree very few of the bytes in the above example look like printable EBCDIC characters.

These fields look nothing like EBCDIC.

A Counter?

I would expect most counters to not be close to exhausting the field’s range. So I would expect the top bits to not be set. Our above example is close to wrapping.

While these values tend to have something like ‘2x’ for the top byte they don’t look like “unsaturated” counters.

So they’re not likely to be counters.

A STCK Value?

I put some sample values into a STCK formatter on the web. I got credible values – dates in 2020, 2021, and 2022.

For the example above I get “07-Mar-2021 06:34:00 ” – which is a very believable date.

So this seems like the best guess by far.

How Do We Interpret This Timestamp?

If we accept these fields are timestamps how do we interpret them?

My view is that this timestamp represents when the structure was allocated, possibly for the first time but more likely a reallocation. (And I can’t see which of these it is.)

Why might this happen?

I can think of a few reasons:

• To move the structure to a different coupling facility. This might be a recovery action.
• To restart the coupling facility. This might be to upgrade to a later CFLEVEL. Or indeed a new machine generation.
• To resize the structure. This is a little subtle: I wouldn’t think, in general, you would reallocate to resize unless you were having to raise the structure’s maximum size.

One thing I’m not sure about is whether there is a time zone offset from GMT. I guess we’ll see what appears credible. I will say that hours and minutes are slightly less important in this than dates. I’m definitely seeing what looks like application-oriented changes such as MQ shared message queue structures appearing to pop into existence.

Conclusion

Guessing field formats is fun, though it is far from foolproof.

I’m a little tentative about this. As with many such things I want to see how customers react to me presenting these dates and times. Call it “gaining experience”.

But I do think this is going to be a useful technique – so I’ve built it into my tabular reporting that lists structures.

As always, more on this when I have something to share.

Making Of

I’m experimenting with the idea that somebody might be interested in how this blog post was made.

The original idea came from a perusal of the SMF 74-4 manual section. It was written in chunks, largely on one day. Two short train journeys, two short tube journeys and a theatre interval yielded the material. It seemed to pour out of my head, and the structure very naturally emerged. Then a little bit of finishing was required – including researching links – a couple of weeks later.