You need to pay the most attention to CPU, memory, and storage space usage.
To stay aware of these metrics, you can configure the system to report alarms to Cloud Eye as needed. You can then take measures to clear any reported alarms.
Configure RDS to report alarms to Cloud Eye if its CPU, memory, or storage utilization reaches a specific value (for example, 90%) a certain number of times (for example, 3 times) within a set period (for example, 5 minutes).
For details on Cloud Eye alarm configuration, see the "Alarm Rule Management" section in the Cloud Eye User Guide.
For details, see section Changing the CPU or Memory of a DB Instance.
For details, see section Scaling Up Storage Space.