My question concerns only Linux software RAID ("md").
I'm going to set up a RAID1 array with three devices. I need to choose between using the third disk as a spare or as an active member of the array. There are no other arrays in the system, so there is no possibility of sharing the spare.
What's the performance impact of the third disk being routinely active? For example, what is the effect on tail latency?
If I go for three active devices, is there a data resilience advantage (e.g. can I scrub the array to take a three-way vote on the correct value of each byte? Apart from mitigating the risk of failure of a second device during reconstruction, is there any other reliability difference?