fastcgi cache path keys zone=name:size

[beatnut]

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 ?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249973,250043#msg-250043