title: Hadoop speculative task execution
body: <p>In Google's MapReduce paper, they have a backup task, I think it's the same thing with speculative task in Hadoop. How is the speculative task implemented? When I start a speculative task, does the task start from the very begining as the older and slowly one, or just start from where the older task has reached(if so, does it have to copy all the intermediate status and data?)</p>