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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?

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3

I can't really speak for BTRFS, but for Linux mdadm software RAID, it supports RAID5 on two drives. However that is only meant as a transitional stage to allow for growing RAID1 to RAID5.

In other words, RAID5 on two drives has the same physical layout as RAID1, just declared differently (even if a two drive RAID5 were considered to have parity, the parity would just happen to be identical to the data so nothing changes on disk).

It is RAID5 in name only and only exists for technical reasons.

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  • Thanks! In the absence of anything else coming up in the next few days, I'll accept this. I hadn't considered transitional (or even degraded states). My only counter would be, why would they not consider RAID1 configurable for one drive? Attempting to answer that myself, you can readily convert a single unraided drive to RAID1 on a whim? Thanks again for your answer :)
    – Madivad
    Feb 12, 2016 at 14:50
  • @Madivad if no answers come up here you could ask the btrfs mailing list and post their answer here. - With mdadm it is possible to do a 1 disk RAID1 but only using --force. I use this technique for mirroring a SSD drive to a write-mostly HDD once a week.
    – frostschutz
    Feb 12, 2016 at 15:40

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