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• System logfiles produced by Hadoop are stored in $HADOOP_INSTALL/logs by default • Each Hadoop daemon running on a machine produces two logfiles. • The first is the log output written via log4j. This file, which ends in .log, should be the first port of call when diagnosing problems, since most application log messages are written here. • Old logfiles are never deleted, so you should arrange for them to be periodically deleted or archived, so as to not run out of disk space on the local node. • The second logfile is the combined standard output and standard error log. This logfile, which ends in .out, usually contains little or no output, since Hadoop uses log4j for Logging. Only the last five logs are retained. Old logfiles are suffixed with a number between 1 and 5, with 5 being the oldest file.

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