It's been my understanding that in order to have a RAID5 setup, one would need to incorporate a minimum of three drives. Basically (and probably poor paraphrasing) two blocks of data and a block of parity distributed across the three drives. Remove one drive from the system and you still have a usable system because the missing block can be recreated from the parity, or the parity can be recreated from the missing block.
My understanding has always been
RAID LEVEL MINIMUM DRIVES 0 2 1 2 5 3 6 4 10 4
Wikipedia article confirming my thoughts on minimum drive requirements: https://en.wikipedia.org/wiki/Standard_RAID_levels#Comparison
Yet on https://btrfs.wiki.kernel.org/index.php/Manpage/mkfs.btrfs#PROFILES it's quoted as being:
RAID LEVEL MINIMUM DRIVES 0 2 1 2 5 2 * 6 3 * 10 4
*These are the values I'm disputing
I haven't been able to find any reasoning as to why this would be the case and any different from RAID5/6 anywhere else. Is it just a typo?