nginx when configured as a reverse proxy seems to utilise two directories

a) nginx http client request body temporary files: $prefix/client_body_temp b) nginx http proxy temporary files: $prefix/proxy_temp

The assumption I'm making is that nginx buffers the client body in the directory when client_body_size is greater than client_body_buffer_size and on the response writes out the response to the directory if the response size is greater than proxy_buffers

Also, can the above directories be placed on tmpfs (memory filesystems). If so, are there any guidelines on sizing the filesystem