Calling update_requirements() might be an expensive op. We can actually skip elaborate(), compute_diff() and most of the calculate_ops. May be we can just do
- Create end addtl_fs_gb usage records for old_untracked if needed
- Update new addtl_fs_gb in the group instance object(s)
- Create begin addtl_fs_gb usage recods for new_untracked if needed
I think this will be clean and efficient.