We are only just at 4.12 kernel as of current testing channel release:
But I agree that the inclusion of space_cache=v2 is a good idea:
This is something I can’t agree with however. We essentially follow the lead re btrfs devs on all defaults so upon our elrepo kernel-ml version adopting this by default then as do we. I don’t think it is wise to deviate from the developer ascribed defaults. Apologies if I have the wrong end of the stick here.
Hope that helps and thanks for your ongoing contribution.