When do you replace old hard drives in a raid6?

[Ram Ramesh <rramesh2400@xxxxxxxxx>]

I am curious if people actually replace hard drives periodically because 
they are old or out of warranty. My 5 device raid6 has several older 
drives (3/5 are 3+ years old and out of warranty) They seem fine with 
SMART and raid scrubs. However, it makes me wonder when they will die. 
What is the best policy in such situations? More importantly, do people 
wait for disks to die and then replace or have some ad hoc schedule of 
replacing (like every 6mo replace oldest) to keep things safe?

Regards
Ramesh

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