for the highest performance, ideally will have a lot of free-space to use for page cache to minimize reading from disk

this can be pretty expensive though to have like a 1:3 or 1:4 ratio of ‘free’ memory to segment cache size

and the more the amount of ram to disk ratio gets away from that the less important having that free space becomes because like you said, you still end up reading from disk

it is sometimes common to use tiers to have a faster ‘hot’ tier with smaller segment caches and only load a limited set of data that is more commonly queried and then a denser tier that loads everything as a failover/less commonly queried stuff, but that pattern doesn’t work for all use cases, more typically when you have some recent data like last week or last month that is more frequently queried and then the slower tier also holds the much older data