[jira] [Created] (HDFS-15273) CacheReplicationMonitor hold lock for long time and lead to NN out of service

["Xiaoqiao He (Jira)" <ji...@apache.org>]

Xiaoqiao He created HDFS-15273:
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             Summary: CacheReplicationMonitor hold lock for long time and lead to NN out of service
                 Key: HDFS-15273
                 URL: https://issues.apache.org/jira/browse/HDFS-15273
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: caching, namenode
            Reporter: Xiaoqiao He
            Assignee: Xiaoqiao He


CacheReplicationMonitor scan Cache Directives and Cached BlockMap periodically. If we add more and more cache directives, CacheReplicationMonitor will cost very long time to rescan all of cache directives and cache blocks. Meanwhile, scan operation hold global write lock, during scan period, NameNode could not process other request.
So I think we should warn this risk to end user who turn on CacheManager feature before improve this implement.
{code:java}
  private void rescan() throws InterruptedException {
    scannedDirectives = 0;
    scannedBlocks = 0;
    try {
      namesystem.writeLock();
      try {
        lock.lock();
        if (shutdown) {
          throw new InterruptedException("CacheReplicationMonitor was " +
              "shut down.");
        }
        curScanCount = completedScanCount + 1;
      } finally {
        lock.unlock();
      }

      resetStatistics();
      rescanCacheDirectives();
      rescanCachedBlockMap();
      blockManager.getDatanodeManager().resetLastCachingDirectiveSentTime();
    } finally {
      namesystem.writeUnlock();
    }
  }
{code}



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