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Determines if, when the prefix of the key used for intermediate statistics collection exceeds a certain length, a hash of the key is used instead. If the value < 0 then hashing is never used, if the value >= 0 then hashing is used only when the key prefixes' length exceeds that value. The key prefix is defined as everything preceding the task ID in the key. For counter type statistics, it's maxed by mapreduce.job.counters.group.name.max, which is by default 128.

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