Comment by johnmaguire

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

It's not just inefficient, you literally can't scale larger. Mirroring is all that RAID 1 allows for. To scale, you'd have to switch to RAID 10, which doesn't allow two-disk fault tolerance (you can get lucky if they are in different stripes, but this isn't fault tolerance.)

But you're right - RAID 1 also scales terribly compared to RAID 6, even before introducing striping. Imagine you have 6 x 16 TB disks:

With RAID 6, usable space of 64 TB, two-drive fault tolerance.

With RAID 1, usable space of 16 TB, five-drive fault tolerance.

With RAID 10, usable space of 32 GB, one-drive fault tolerance.