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Sometimes performance issues or errors don't originate in your application, but the host it runs on. Since gem version 1.2 we track host metrics (if enabled by you) and show them in the Hosts section. As of today we also show a snapshot of host metrics on all error and performance sample pages, for the host that handled that specific request.
This way you can easily track if an error occurred because of swapping, if your request was slow because of high disk I/O, or one of the other three metrics we keep track of for every request.