For a non-degraded RAID 6 array, why do most implementations and documentation require at least 4 drives? Why can't it have 3 disks, for 1 data and 2 parity blocks per stripe? Why force a minimum of 2 data and 2 parity?
- you can make a RAID 6 with 3 real and 1 missing disk with mdadm but that's not what I'm talking about
- a 3 disk RAID 6 will result in only 1/3 of the storage capacity of the bare disks.
- a 3 disk RAID 6 would be functionally similar to a 3 disk RAID 1 array. Though I wonder if mdadm's check sync can find and correct bit rot on a 3 disk raid 1. But that's a topic for another question.
The reason I'm asking is because I'm looking at changing my home DIY NAS setup to be similar to the hybrid RAID in store bought NAS boxes that offer dual disk redundancy with differently sized disks. So using multiple mdadm RAID arrays as physical volumes in a single logical volume, as described here but with RAID 6's and 3 disk RAID 1's.
It would just seem more consistent if all of the RAIDs that make up the dual disk redundancy LVM were all RAID 6's.