fastcgi_busy_buffers_size

When buffering of responses from the FastCGI server is enabled, limits the total size of buffers that can be busy sending a response to the client while the response is not yet fully read. In the meantime, the rest of the buffers can be used for reading the response and, if needed, buffering part of the response to a temporary file. By default, size is limited by the size of two buffers set by the fastcgi_buffer_size and fastcgi_buffers directives.