When people start digging into AWS RDS monitoring, there’s usually questions around these three metrics. What do they mean and how should we be using them?
Free Memory: This is just unused RAM. If it’s high, your instance might be over provisioned. If it’s low, it doesn’t necessarily mean a problem - it could just mean the memory is being used efficiently.
Active Memory: This is what’s actually being used by running processes. It gives you a sense of how much memory your DB engine and other processes are consuming in real-time.
Freeable Memory: This one is one to watch more closely. It includes both free memory and memory used for caching that can be reclaimed. Low freeable memory means your instance is tight on RAM, and you might start seeing performance issues.
If Free Memory is low but Freeable Memory is high, you’re fine - the system is just using RAM efficiently for caching.
If both are low, you might be headed for trouble.
Queries could slow down, and you might need a bigger instance.
If Active Memory is way below the instance’s total RAM, you could be overpaying for capacity you don’t need.
Bottom line: Free Memory isn’t the whole picture - pay closer attention to Freeable Memory instead.