[jira] [Created] (HDFS-6784) Avoid rescan twice in HDFS CacheReplicationMonitor for one FS Op if it calls setNeedsRescan multiple times.

["Yi Liu (JIRA)" <ji...@apache.org>]

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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