Starting with 7.0, the cluster compat version is saved in chronicle. As a result upgrades to ns_config are not allowed. There is a window between the cluster compat mode getting updated and the ns_config getting update. Should a failure occur between the two the cluster compat mode would not reflect the content of ns_config. Also a client which "gets" a value from ns_config after seeing the cluster compat mode get bumped might get the older, pre-upgrade, value.