db_cache_advice can be set to either of on | This is the default if statistics_level is typical or all | |
ready | The advisory is turned off but the memory is allocated (so v$db_cache_advice can still be quried) | |
off | This is the default if statistics_level is basic |
db_cache_advice to on is a prerequisite for v$sga_target_advice to be populated (the other one is to set statistics_level to at least typical).