Explanation:
When proxying an HTTP request, nginx first fills up the memory buffers (set by proxy_buffer_size and proxy_buffers).
When these are full, it then writes them to a temporary file in batches of proxy_temp_file_write_size until it reaches proxy_max_temp_file_size.
When proxy_max_temp_file_size is not set, and a very large file is being served, it reaches the maximum of 1GB, and nginx begins to behave weirdly.
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