Hi, My Linux guy has asked a question (one of many) I can't answer about Linux memory/swap storage hierarchy and how VM affects that.
Within Linux (forget VM for a moment), Linux tries to keep active pages in memory. When Linux completely consumes memory, it migrates what it considers to be inactive pages to swap to free memory. When it needs pages from swap, it migrates them back to memory, sometimes resulting in other pages being migrated to swap. Now put VM back into the picture: VM manages both Linux' memory and the vdisk used to back swap using the storage hierarchies available to it: central storage, expanded storage and paging disk. Using the IND USER EXP command and my performance monitor (assuming I'm interpreting them both correctly), I can see that both Linux machine virtual storage and swap/vdisk are spread across all three of the VM storage hierarchies. Now for the question: Since Linux has moved what it considers inactive pages to swap/vdisk, why would VM keep any of that machine's vdisk pages in central storage in a VM-central-storage-constrained environment? I would expect that VM would migrate all of a Linux machine's vdisk pages to xstor/disk before it would migrate any of that machine's virtual storage pages away to xstor/pagdisk. The only exception I can think of would be that VM might migrate vdisk pages from pagdisk/xstor to cstor as part of Linux swap migration back to its memory. And, another question: when VM decides to migrate pages from expanded storage to dasd, does it choose vdisk pages before virtual storage pages? Thanks in advance for any answers you can provide. Dennis Mutual of Omaha
