[hooli_gan]

RAID5/6 is not needed with btrfs. One should use RAID1, which supports striping the same data onto multiple drives in a redundant way.

[johnmaguire]

How can you achieve 2-disk fault tolerance using btrfs and RAID 1?

[Dalewyn]

By using three drives.

RAID1 is just making literal copies, so each additional drive in a RAID1 is a self-sufficient copy. You want two drives of fault tolerance? Use three drives, so if you lose two copies you still have one left.

This is of course hideously inefficient as you scale larger, but that is not the question posed.