Please note, total memory is not the summation of free and active memory of your Aurora Postgres instances. In Aurora Postgres, I/O is handled by the Aurora storage driver. There is no file system or secondary level of caching for tables or indexes. This means that shared_buffers should be larger than what the PostgreSQL community recommends.
For Aurora DB instances, the default value of the DB parameter group shared_buffers is set between 50% and 75%, depending on the instance class. This is because Aurora PostgreSQL eliminates double buffering and doesn't utilize file system cache. As a result, Aurora PostgreSQL can increase shared_buffers to improve performance. It's a best practice to use the default value of 75% for the shared_buffers DB parameter when using Aurora PostgreSQL. A smaller value can degrade performance by reducing the available memory to the data pages while also increasing I/O on the Aurora storage subsystem.
You can read more about memory and buffer related parameters in Aurora Postgres here: https://aws.amazon.com/blogs/database/amazon-aurora-postgresql-parameters-part-1-memory-and-query-plan-management/#:~:text=Memory%20and%20buffer%20related%20parameters
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