Where did we leave off? We were talking about models and how a model like an E10 has 10 PUs potentially available to the customer. We said potentially because how many you actually have available is limited by how many PUs you actually purchased.
So now we’re ready to wrap this section up by talking about the “capacity setting.”
To make sense of this, you need to know one thing: a PU does not necessarily run at full speed. Or, to put it another way, it’s possible to “turn down” a PU so that it runs at less than its full speed. Why would you want to do that? Well, actually it’s a way of offering an amazing level of granularity to the customer. Whatever “capacity” you need to handle your workload (a number obtained by means of your “capacity planning process”), you can find a z10 that is a pretty close match. That way, you don’t have to buy excessive capacity. You can buy what you need.
Now, here is where we are – we know that we can buy a z10 with some number of PUs (a number less than or equal to the number of PUs available in the selected model) and we know that we can set the speed of the PUs (on some models) to something less than full speed. What we need is a way to express those two things: the PU quantity and the PU speed.
The capacity setting is how you express those quantities. The format differs between the BC machines and the EC machines. We’ll look at both.
z10 EC capacity settings
This one is easy. The capacity setting for the EC machine is composed of three numbers. The first number is either a 4, 5, 6, or 7. A 7 indicates that the PUs are running at full speed. A 6 indicates that the PUs are turned down “one notch.” A 5 indicates two notches. And 4 is the lowest speed to which a PU can be turned down.
How much is a “notch”? I don’t have an official number to quote to you. But, just for talking purposes, I might equate them to something along these lines:
7 is 100%
6 is 70%
5 is 50%
4 is 25%
Those are my numbers, not official IBM numbers, just to give you a rough sense of how the settings might relate.
So that’s the first number.
The next two numbers represent how many general purpose PUs you are buying. (I’ll explain the general purpose qualifier in another post).
We can now specify an EC machine like this:
A 2097-E12 701 is a z10 EC machine with one general purpose PU running at full speed.
A 2097-E12 405 is a z10 EC machine with five general purpose PUs running at a subcapacity of 4 (not at full speed).
Get the idea?
One note: This applies up to a 12-way z10 EC. Once you reach 13-way or higher, the PUs always run at full speed (i.e., 7). There is no capacity setting of 413, 513, or 613. From 13-way on up, it always starts with a 7, e.g., 713, 714, on up to 764.
z10 BC capacity settings
Let’s look now at the z10 BC. Naturally, it has to be a little different.
We still have three characters to specify the capacity and the number of PUs. The first character, though, is alphabetical, ranging from A to Z. A is the slowest; Z is the fastest. A PU running at a setting of Z has, let’s say, perhaps 26 times the capacity of a PU running at a setting of A.
The next two digits represent the number of general purpose PUs. Note: On the z10 BC machine (which has a model of E10, you may recall), the most general purpose PUs you can have is five. In other words, the BC can be configured up to a 5-way machine.
Thus, the entry point for a z10 BC has a capacity setting of A01, which is a 1-way or “uniprocessor.” A machine with an A capacity can be expanded to be a 2-way, 3-way, 4-way, or 5-way (A02, A03, A04, and A05, respectively).
Notice, then, that both the BC and the EC can be grown (or “scaled”) horizontally (add more PUs), vertically (move to a faster PU speed), or both.
And this completes our look at three key numbers that describe a mainframe:
- Machine type
- Model number
- Capacity setting
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