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What are the three metrics of AWS RDS: Free Memory (Enhanced monitoring), Active Memory (Enhanced monitoring), and Freeable Memory (CloudWatch monitor)?
What is the relationship between them?
Look at these two pictures.
The value of three metrics are different.
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Freeable memory is the amount of memory (RAM) available on the RDS instance. The total amount of memory available on an Amazon RDS instance depends on the instance class. For example, the instance class db. r5. 8xlarge provides 32 vCPU and 256 GiB memory.
In Amazon RDS for MySQL, you can monitor four memory statuses: Active: The memory that's actively being consumed by database processes or threads. Buffer: A buffer is a temporary space in memory that's used to hold a block of data. Free Memory: The memory that's available for use.
If your freeable memory remains consistently low, you can change your RDS DB instance size to a larger instance size that has more memory. To monitor swap memory, turn on Enhanced Monitoring to review metrics in intervals with a granularity of as little as one second.
Let me answer your question in two parts.
As per official guide
Amazon RDS Enhanced Monitoring — Look at metrics in real time for the operating system.
Amazon CloudWatch Metrics – Amazon RDS automatically sends metrics to CloudWatch every minute for each active database. You are not charged additionally for Amazon RDS metrics in CloudWatch.
Meaning, enhanced monitoring allows your to monitor operating system counters while cloudwatch monitoring enables your to monitor performance counters per database instance.
Enhanced monitoring info source
Free Memory
The amount of unassigned memory, in kilobytes.
Active Memory
The amount of assigned memory, in kilobytes.
Freeable Memory Official Source
The amount of available random access memory.
Units: Bytes
Freeable memory is not a indication of the actual free memory available. It is the memory that is currently in use that can be freed and used for other uses; it's is a combination of buffers and cache in use on the database instance.
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