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https://issues.apache.org/jira/browse/HDFS-14579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16866612#comment-16866612
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Kihwal Lee commented on HDFS-14579:
-----------------------------------
{{node.getResolvedAddress()}} does not cause an actual look up. The ctor of
{{InetSocketAddress}} calls {{InetAddress.getByName()}}, which only verifies
the format as what's passed in is the string representation of an IP address.
{code:java}
/**
* Determines the IP address of a host, given the host's name.
*
* <p> The host name can either be a machine name, such as
* "{@code java.sun.com}", or a textual representation of its
* IP address. If a literal IP address is supplied, only the
* validity of the address format is checked.
...
*/
public static InetAddress getByName(String host)
{code}
> In refreshNodes, avoid performing a DNS lookup while holding the write lock
> ---------------------------------------------------------------------------
>
> Key: HDFS-14579
> URL: https://issues.apache.org/jira/browse/HDFS-14579
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 3.3.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: HDFS-14579.001.patch
>
>
> When refreshNodes is called on a large cluster, or a cluster where DNS is not
> performing well, it can cause the namenode to hang for a long time. This is
> because the refreshNodes operation holds the global write lock while it is
> running. Most of refreshNodes code is simple and hence fast, but
> unfortunately it performs a DNS lookup for each host in the cluster while the
> lock is held.
> 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.
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