So if I understand you correctly:
- You want to know how much of used memory is for “current” working sets.
- You want to be able to tell if your “current” or “near future” working sets will fit in ram
Can you please elaborate on what info would lead you to conclude more RAM is needed?
The sections db.serverStatus().wiredTiger.capacity and db.serverStatus().wiredTiger.cache seem to contain metrics that will measure activity levels (recent / current) but they don’t necessarily mean “buy more ram” since this is related to working sets and expected future load.
So are you looking to get some reading of “current/recent working set sizes”?