This entry was originally published on 2014-07-04.

By Garth Jones.

It is a common belief that Db2 11 for z/OS can only use 1MB size pageable large pages, other than for buffers, if the CEC has Flash Express installed. For example, the text for APAR PM85944 infers just that. However, this is not true.

The only requirement is that the CEC be SCM-capable (SCM = Storage-Class Memory). In other words, it does not matter whether Flash Express is actually installed or not. So if a customer is running on a zEC12 CEC without Flash Express, Db2 could request, and be given, a 1MB size pageable large page, residing in a 1MB size large page frame.

However, if that page needs to get paged out for some reason, at that point it will be broken down into 4KB page frames. To put it another way, pageable large pages are available on a zEC12 capable CEC with the caveat that if Flash Express is not installed, then if those pages are ever paged out they will be demoted to 4KB page frames.

1MB pageable large pages that are demoted to 4KB page frames as they paged out will never be coalesced: that is, they will remain 4KB page frames for the remaining life of the IPL.