How does nginx cache work ?

[ujaeek at yahoo.co.jp]

Hi, 

Currently we are modify some nginx proxy cache functions, and I would like to know how nginx cache work?

1. does nginx cache data in memory or read from disk everytime?

for example, first I do `GET /example.jpg`, nginx will cache it and save to disk.
next time I do `GET /example.jpg`, does nginx read the file from disk then send to client, 
or nginx just send the file from memory to client because nginx has cached it in memory ?

2. if nginx save it in memory, what happens if the file is 1GB?

does nginx malloc a 1GB memory and save the file? or split the file into many parts, and save it separately using 
something like array or linked list?

Thanks
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