An often missed feature is the seed device. Any Btrfs volume can be flagged as a read-only seed. Mount it, you can add a 2nd device, and now re-mount. It's a un

An often missed feature is the seed device. Any Btrfs volume can be flagged as a read-only seed. Mount it, you can add a 2nd device, and now re-mount. It's a union filesystem like thing. Changes only go to the 2nd device. If you used 'btrfs remove" on the 1st device, data is migrated to the 2nd device and the 1st is removed, so it replicates the filesystem. The seed setting can be applied to multiple device volumes. So you can add this 2nd device, make that volume a read-only seen, and add a 3rd device for read-write, and so on.