I found similar information about :

limit_req_zone $binary_remote_addr zone=one:1m "One megabyte zone can keep about 16 thousand 64-byte states. "

or

ssl_session_cache shared:SSL:1m; " The cache size is specified in bytes; one megabyte can store about 4000 sessions."

but there is no info about fastcgi_cache_path keys_zone shared memory.

I was searching any tip in the source code of fastcgi module with no success.

I'm trying to adjust my setup. There will be about 30k of cached files. How big should be size of shared memory ?