(Originally posted 2019-01-14.)
In Paging Subsystem Design, in 2005, I opened with the words
"Periodically on IBM-MAIN I’m caused to revisit something. With the advent of z990/z890 and “supposedly abundant” 🙂 real memory it seems to be time to revisit paging subsystem design.”
Note the smiley is in the original. Seems funnier now.
The z990/z890 era seems forever ago, now. And some things have changed. So an excuse to write about paging subsystem design would be welcome.
Well, I got asked a question about it by a customer. The more I thought about it the less sense it made just to reply privately. Instead, here’s a follow-on blog post.
So let’s assume z13/z141 era, there being slight differences between the two.
With z13, it is possible to acquire IBM Flash Express cards (actually available with zEC12 as well). You can page to Flash, and it’s highly performant.
With z14, Virtual Flash Memory (VFM), which is a use of real memory outside of an LPAR’s normal addressing range.
(The analogy with Expanded Storage is quite good. Indeed later implementations of Expanded Storage were in real memory. But that’s not something I want to dwell on here.)
In the rest of this, I’m going to use the term “Flash” to cover both the z13 and z14 implementations.
The Operative Question
If I have Flash, do I need as much of a paging subsystem as if I didn’t? That is the question. (What isn’t the question is whether I could have less real storage if I have Flash to page into.)
Of course “as much of a paging subsystem” refers to both space and I/O bandwidth considerations.
Assuming sufficient Flash, this becomes an availability question. I make this assumption because if it’s true we’re not going to see significant paging to the page data sets on disk. It’s actually a pretty fair assumption, for all but the very biggest workloads. For example, you can have up to 6 Terabytes of VFM (in 1.5TB increments).
But suppose something happened to Flash. You would, ideally, want the paging subsystem to handle the load. Whether the LPARs using it survived is another matter, of course; For instance, would major address spaces ABEND?
So this is a matter of attitude to risk. If you think the event is ludicrously unlikely, or that the event that caused the loss of Flash would also bring down the LPARs using it, maybe you don’t need much paging disk capability to back it up.
If, however, you absolutely must have a backup in the case of a Flash failure you would ideally configure the paging subsystem as if Flash weren’t there. Which would mean following the practices alluded to in Paging Subsystem Design.
But that’s the ideal. In practice, people might well configure somewhere between the two extremes. Now to talk to the customer about where they are on the spectrum. And to see whether I missed the point or not. 🙂
I’m using “z13” and “z14” to denote generations. Of course, z13s and z14 ZR1 behave the same way as their larger cousins. ↩