Mind The Gap

Or “An Indecent Interval”?

I’ve written about RMF intervals before – when writing about System Recovery Boost.

I suppose one lesson of this post is to not assume that all intervals are of equal length. We’ve got pretty used to assuming that a system’s RMF interval is, say, 15 minutes long. We’ve maybe got used to it coinciding with the SMF interval – though that is a bit of a tenuous assumption.

Maybe another lesson is that there can be gaps in the data.

Gap Analysis

Recently I’ve made progress in analysing gaps in the data customers send me. By a gap I mean a period of time for which there is no RMF interval.

Having a gap can sometimes do strange things to calculations. For example, working out CPU utilisation by summing up the CPU used over, say, an hour and dividing by the number of seconds would underestimate – if an interval were missing.

Gaps can be benign, for example if the system were down for a few minutes. (Though an unintended bounce of a system is not what I’d actually call benign.)

Often, though, a gap in the SMF data a customer sends to me is caused by a problem in their handling the SMF data. (Conversely, I quite often see duplicate data.) If I see either of these I tend to point them out to the customer.

Irregularity

I’m also observing that sometimes the interval length is irregular.

The most frequent cause of this is System Recovery Boost or Recovery Process Boost. Almost as often, though, it’s a WLM policy switch that causes intervals to terminate. (You might expect that one to be regular – as most policy switches happen on a schedule.)

Minor irregularity can happen on a seriously overloaded system. Ultraminor – a second or two – is very common and doesn’t bother me.

A Decent Interval?

So what would a nice, regular, synchronised interval look like?

It would be synchronised with the SMF interval, with the actual quarter of an hour, and there wouldn’t be any gaps or duplication.

Conclusion

I think it pays not to assume that there are no gaps in the data, that intervals are all the same length, and that intervals are perfectly synchronised.

Further, it pays to know what the state of the system was during any interval. For example, whether it represented part of a boost period, and of what kind.

And missing intervals require some kind of explanation.

Making Of

I don’t recall when I started this post. I do know I’m finishing it on a plane to South Africa – to see a couple of customers who I hope would consider me to be a friend.

You might consider this one to be mercifully brief, or maybe a little high level. I’d agree with both sentiments.