**Section 1 — The Context (The 'Why')**
MapReduce pioneered large-scale batch processing but suffers from disk I/O at every stage—map writes to disk, shuffle reads and writes, reduce reads. This makes it unsuitable for iterative workloads like ML where the same data is processed repeatedly. A naive use of MapReduce for machine learning causes 10–100x longer runtimes than in-memory frameworks....
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