4 HDDs can do 200-400MB/s _sequential_ IO, 1 modern SSD can do 150-200MB/s _random_ IO and 400MB/s sequential IO while 4 HDDs would have a hard time doing IIRC more than 8MB/s random IO
i don't argue that. It is just random IO isn't that important for our use case, especially compare to capacity and sequential IO which are important for us and which HDD is ok for.
