> I can do a RAID-1 (mirror) with 3 drives on ZFS or mdadm and lose 2 drives.
You can do this with BTRFS also; it's called RAID1C3 (for three copies). RAID1C4 is also an option with larger arrays.
The issue is that if you have, say, four drives and standard RAID-1 (two copies) it doesn't divide the drives into mirrored pairs where you could lose one drive from each pair without any ill effect (like RAID-10), but rather just ensures that each piece of data is stored on any two different drives. With 4-drive RAID-10 if you lose two drives you have a 50% chance of losing the whole array, since both failing drives could be from the same pair. With BTRFS RAID1 in a four-drive array you have a high probability of losing both copies of some of the data no matter which two drives fail.
RAID-6 is strictly better than either, though, since you can lose any two drives and still access all your data.
You can do this with BTRFS also; it's called RAID1C3 (for three copies). RAID1C4 is also an option with larger arrays.
The issue is that if you have, say, four drives and standard RAID-1 (two copies) it doesn't divide the drives into mirrored pairs where you could lose one drive from each pair without any ill effect (like RAID-10), but rather just ensures that each piece of data is stored on any two different drives. With 4-drive RAID-10 if you lose two drives you have a 50% chance of losing the whole array, since both failing drives could be from the same pair. With BTRFS RAID1 in a four-drive array you have a high probability of losing both copies of some of the data no matter which two drives fail.
RAID-6 is strictly better than either, though, since you can lose any two drives and still access all your data.