What's your starting array look like? If you're already on Btrfs then I recall you could do something like `btrfs balance -d raid6 -m raid1c3 /`
I'll have a look, thanks! I guess failing this will make me test by backup strategy that I have never tested in the past.
For btrfs specifically there is an online calculator [1] that shows you the effective capacity for any arbitrary configuration. I use it whenever I add a drive to check whether it’s actually useful.
1: https://carfax.org.uk/btrfs-usage/?c=2&slo=1&shi=1&p=0&dg=1&...