[Joe_Cool]

I believe you are right. You can only convert back to the metadata from before. So any new or changed (different extents) files will be lost or corrupted.

So best to only mount ro when considering to rollback. Otherwise it's pretty risky.

[heftig]

No, it also covers the data. As long as you don't delete the rollback subvolume, all the original data should still be there, uncorrupted.

Even if you disable copy-on-write, as long as the rollback subvolume is there to lay claim to the old data, it's considered immutable and any modification will still have to copy it.

[Joe_Cool]

I understood it as "it doesn't touch/ignore the data". But I guess we mean the same thing.

You are right. All of the old files will be in areas btrfs should consider used. So it should correctly restore the state from before the migration. Thanks!