By the sounds of it, a cluster may be configured for
the 100 PG / OSD target; adding pools to the former configuration scenario will require an
increase in OSDs to maintain that recommended PG distribution target and accommodate an
increase of PGs resulting from additional pools.
To be clear, 100 or 200 or any other number is a target, not a precise goal. In the past
one could increase the number of PGs in a given pool but not decrease; with Nautilus we
got the ability to decrease.
So if you forsee adding more pools, you either just add them and the effective ratio
increases a bit, or adjust some existing pools down. Adding OSDs does have the effect of
reducing the number of PGs per (absent other topology changes at the same time). So if
you think you’re going to be adding pools, maybe aim lower rather than higher.