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From "Stephen O'Donnell (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-14563) Enhance interface about recommissioning/decommissioning
Date Fri, 14 Jun 2019 16:54:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-14563?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16864264#comment-16864264
] 

Stephen O'Donnell commented on HDFS-14563:
------------------------------------------

Looking at the source, it seems it would be feasible to take the DNS lookup out of the write
lock.

Right now, it calls:

{code}
  public void refreshNodes(final Configuration conf) throws IOException {
    refreshHostsReader(conf);
    namesystem.writeLock();
    try {
      refreshDatanodes();
      countSoftwareVersions();
    } finally {
      namesystem.writeUnlock();
    }
  }
{code}

The line refreshHostsReader(conf); reads the new config file and does a DNS lookup on each
entry - the write lock is not held here. Then the main work is done here:

{code}
  private void refreshDatanodes() {
    final Map<String, DatanodeDescriptor> copy;
    synchronized (this) {
      copy = new HashMap<>(datanodeMap);
    }
    for (DatanodeDescriptor node : copy.values()) {
      // Check if not include.
      if (!hostConfigManager.isIncluded(node)) {
        node.setDisallowed(true);
      } else {
        long maintenanceExpireTimeInMS =
            hostConfigManager.getMaintenanceExpirationTimeInMS(node);
        if (node.maintenanceNotExpired(maintenanceExpireTimeInMS)) {
          datanodeAdminManager.startMaintenance(
              node, maintenanceExpireTimeInMS);
        } else if (hostConfigManager.isExcluded(node)) {
          datanodeAdminManager.startDecommission(node);
        } else {
          datanodeAdminManager.stopMaintenance(node);
          datanodeAdminManager.stopDecommission(node);
        }
      }
      node.setUpgradeDomain(hostConfigManager.getUpgradeDomain(node));
    }
  }
{code}

All the isIncluded(), isExcluded() methods call node.getResolvedAddress() which does the DNS
lookup. We could probably change things to perform all the DNS lookups outside of the write
lock, and then take the lock and process the nodes. Also change or overload isIncluded() etc
to take the inetAddress rather than the datanode descriptor.

It would not shorten the time the operation takes to run overall, but it would move the long
duration out of the write lock and avoid blocking the namenode for the entire time.

> Enhance interface about recommissioning/decommissioning
> -------------------------------------------------------
>
>                 Key: HDFS-14563
>                 URL: https://issues.apache.org/jira/browse/HDFS-14563
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client, namenode
>            Reporter: He Xiaoqiao
>            Assignee: He Xiaoqiao
>            Priority: Major
>
> In current implementation, if we need to decommissioning or recommissioning one datanode,
the only way is add the datanode to include or exclude file under namenode configuration path
then execute command `bin/hadoop dfsadmin -refreshNodes` and trigger namenode to reload include/exclude
and start to recommissioning or decommissioning datanode.
> The shortcomings of this approach is that:
> a. namenode reload include/exclude configuration file from devices, if I/O load is high,
handler may be blocked.
> b. namenode has to process every datnodes in include and exclude configurations, if there
are many datanodes (very common for large cluster) pending to process, namenode will be hung
for hundred seconds to wait recommision/decommision finish at the worst since holding write
lock.
> I think we should expose one lightweight interface to support recommissioning or decommissioning
single datanode, thus we can operate datanode using dfsadmin more smooth.



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