The patch to enable $upstream_cache_status. Possible values:
MISS
EXPIRED expired, request was passed to backend
UPDATING expired, stale response was used due to
proxy/fastcgi_cache_use_stale updating
STALE expired, stale response was used due to
proxy/fastcgi_cache_use_stale
HIT
--
Igor Sysoev
http://sysoev.ru/en/