Yi Liu created HDFS-6784:
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Summary: Avoid rescan twice in HDFS CacheReplicationMonitor for
one FS Op if it calls setNeedsRescan multiple times.
Key: HDFS-6784
URL: https://issues.apache.org/jira/browse/HDFS-6784
Project: Hadoop HDFS
Issue Type: Improvement
Components: caching
Affects Versions: 3.0.0
Reporter: Yi Liu
Assignee: Yi Liu
In HDFS CacheReplicationMonitor, rescan is expensive. Sometimes,
{{setNeedsRescan}} is called multiple times, for example, in
FSNamesystem#modifyCacheDirective, there are 3 times. In monitor thread of
CacheReplicationMonitor, if it checks {{needsRescan}} is true, rescan will
happen, but {{needsRescan}} is set to false before real scan. Meanwhile, the
2nd or 3rd time {{setNeedsResacn}} may set {{needsRescan}} to true. So after
the scan finish, in next loop, a new rescan will be triggered, that's not
necessary at all and inefficient for rescan twice.
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