Xinwei Qin created HDFS-8710:
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Summary: Always read DU value from the cached "dfsUsed" file on
datanode startup
Key: HDFS-8710
URL: https://issues.apache.org/jira/browse/HDFS-8710
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: Xinwei Qin
Assignee: Xinwei Qin
Currently, DataNode will cache DU value in "dfsUsed" file termly. When DataNode
starts or restarts, it will read in the cached DU value from "dfsUsed" file if
the value is less than 600 seconds old, otherwise, it will run DU command,
which is a very time-consuming operation(may up to dozens of minutes) when
DataNode has huge number of blocks.
Since slight imprecision of dfsUsed is not critical, and the DU value will be
updated every 600 seconds (the default DU interval) after DataNode started, we
can always read DU value from the cached file (Regardless of whether this value
is less than 600 seconds old or not) and skip DU operation on DataNode startup
to significantly shorten the startup time.
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