The Shuffle Write: Staging Data by Key

A shuffle has two halves, and the first is the write, which happens on the map side, the executors that hold the input data. When a wide operation runs, each of these executors takes its local partition and sorts the rows into buckets, one bucket for each destination partition, based on the key you are grouping or joining by. A row for region EU goes in the EU bucket; a row for APAC goes in the APAC bucket. That bucketing by key is the write. The executor does not send these buckets immediately. It writes them to its own local disk first, as shuffle files. This staging to disk is deliberate: it means the data survives even if the receiving side is not ready yet, and it lets the fetch happen on the reduce side's schedule. It also means the write half of every shuffle pays a full disk write

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