[jira] [Created] (HDFS-14366) Improve HDFS append performance

["Chao Sun (JIRA)" <ji...@apache.org>]

Chao Sun created HDFS-14366:
-------------------------------

             Summary: Improve HDFS append performance
                 Key: HDFS-14366
                 URL: https://issues.apache.org/jira/browse/HDFS-14366
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs
            Reporter: Chao Sun
            Assignee: Chao Sun


In our HDFS cluster we observed that {{append}} operation can take as much as 10X write lock time than other write operations. By collecting flamegraph on the namenode (see attachment), we found that most of the append call is spent on {{getNumLiveDataNodes()}}:

{code}
  /** @return the number of live datanodes. */
  public int getNumLiveDataNodes() {
    int numLive = 0;
    synchronized (this) {
      for(DatanodeDescriptor dn : datanodeMap.values()) {
        if (!isDatanodeDead(dn) ) {
          numLive++;
        }
      }
    }
    return numLive;
  }
{code}
this method synchronizes on the {{DatanodeManager}} which is particularly expensive in large clusters since {{datanodeMap}} is being modified in many places such as processing DN heartbeats.

For {{append}} operation, {{getNumLiveDataNodes()}} is invoked in {{isSufficientlyReplicated}}:
{code}
  /**
   * Check if a block is replicated to at least the minimum replication.
   */
  public boolean isSufficientlyReplicated(BlockInfo b) {
    // Compare against the lesser of the minReplication and number of live DNs.
    final int replication =
        Math.min(minReplication, getDatanodeManager().getNumLiveDataNodes());
    return countNodes(b).liveReplicas() >= replication;
  }
{code}

The way that the {{replication}} is calculated is not very optimal, as it will call {{getNumLiveDataNodes()}} every time even though usually {{minReplication}} is much smaller than the latter. 




 



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