Technical reason behind client_body_temp_path

[attozk]

I am trying to understand the reason for buffering incoming request
(client_body_buffer_size) in which nginx would either keep request in memory
or write to file client_body_temp_path (based on request size).

What are the performance advantages and/or technical challenges to such an
approach as opposed to piping directly (even for smaller requests) to
unbuffered piping to remote server e.g. http://tengine.taobao.org/

Nginx allows disabling upstream (to client) buffering in which response is
sent to the client synchronously while it is receiving it, why not the
opposite is possible? What are the technical challenges/pros/cons of writing
to disk (client_body_temp_path) or in-memory buffer
(client_body_buffer_size)?

Please share your thoughts.

Thanks

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,254968,254968#msg-254968