We have a Aurora MySQL cluster with a primary Writer + 1x Reader running 24x7, the reader load is very low during midnight 1am to 7am.
There's two option here:
Just reserve both writer and reader instance, get the reserved cost
Shutdown the reader from 1am to 7am daily
Reserved savings:
We are running r6g.large for our reader instance in Singapore region:
if r6g.large run 24x7 for 30 days, reserved instance cost:
$0.242 (reserved pricing) x 24 hours x 30 days = $174
Shutdown 6 hours per day
if r6g.large run 18x7 for 30days, on demand instance cost:
$0.313 (on demand) x 18 hours x 30 days= $169.02
Shutdown 7 hours per day
What if we take some risk and start the reader at 8am (1 hour later), work load is still faily low:
$0.313 (on demand) x 17 hours x 30 days= $159.63
Conclusion
It appears the break-even point here is shutting down 6 hours per day, with close to 3% savings compare to reserved instance pricing. The more hour we can afford to shutdown daily, the more savings we have. Shutting down for 7 hours would be a good option for us to go.