On my servers I want some available RAM which means "used - buffers", because this means I configured my servers correctly and nothing is running away, or nothing is using more than it should.
On the other hand, you want "free" almost zero on a warmed up server (except some cases which hints that heaps of memory has been recently freed) since the rest is always utilized as disk cache.
Similarly having some data on swap state doesn't harm as long as it's spilled there because some process has ran away and used more memory than it should be.
So, RAM usage metrics carry a ton of nuance and can mean totally different things depending on how you use that particular server.
I do not enjoy showing up to yet another discussion of why our response times just went up “for no reason”. Learn your latency tables people.
Look, we're using just 50% of that RAM. Look, there're two cores that are almost idle.
No & No. Rest of the RAM is your secret for instant responses, and that spare CPU resource is for me to do system management without you notice or to front the odd torrent of requests we have semi regularly (e.g.: /. hug of death. Remember?).
Cache misses don't have anything to do with memory pressure, they're related to caching effectiveness.
Production systems shouldn't have any page faults because they shouldn't be using swap.
The traditional way Linux memory pressure was measured using a very small swap file and check for any usage of it. Modern Linux has the PSI subsystem.
Also, monitoring for OOM events also means a system needs more RAM or a workload needs to be tuned or spread out.