Hmm that’s for subvolumes, not clear how you’d use that for multiple disks.

I’d rather use bcachefs but that’s not implemented yet :) I’m trying to decide whether I need to implement bcachefs multi-disk myself

Looking at the source of this module, I can make that happen by configuring device1 and adding "-m dup -d dup /dev/device2" as extra mkfs options.

I'm about to press the big red button, so will also ask here:

Quick sanity check!
I have sda and nvme in my machine, sda was created with disko
I backed up everything to sda
Now I want to do nixos-anywhere with disko this machine
Will I get my sda wiped, and how to not get my sda wiped?

the one above is solved
i have a new one
trying to run nixos-install and get:

error:
       … while calling the 'head' builtin

         at /mnt/nix/store/gzf4zwcakda1nykn6h0avh45xhjhvsz4-source/lib/attrsets.nix:960:11:

          959|         || pred here (elemAt values 1) (head values) then
          960|           head values
             |           ^
          961|         else

       … while evaluating the attribute 'value'

         at /mnt/nix/store/gzf4zwcakda1nykn6h0avh45xhjhvsz4-source/lib/modules.nix:809:9:

          808|     in warnDeprecation opt //
          809|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          810|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: The option `fileSystems."/".device' has conflicting definition values:
       - In `/mnt/nix/store/d1g5graq05ksrkr0w4ym6wbg6xgdkps8-source/flake.nix': "/dev/mapper/nvme-crypt"
       - In `/mnt/nix/store/d1g5graq05ksrkr0w4ym6wbg6xgdkps8-source/hosts/immortal/hardware-configuration.nix': "/dev/disk/by-uuid/d5897059-dab4-4e73-8904-ec78689db058"
       Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

as i understand /dev/mapper/nvme-crypt comes from disko
and /dev/disk/by-uuid/d5897059-dab4-4e73-8904-ec78689db058 nixos generated