Alieh - you might want to look at MR concepts (job, input, output etc.).
The short answer is *hadoop* doesn't decide anything, it's upto the MR job to decide input/output etc.
You might want to read through this:
On Feb 3, 2012, at 11:46 PM, Alieh Saeedi wrote:
> 1- How does Hadoop decide where to save file blocks (I mean all files include files written by reducers)? Could you please give me a reference link?
Arun C. Murthy