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Date: Fri, 21 Nov 2008 13:59:44 -0800 (PST)
From: "Konstantin Shvachko (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-4061) Large number of decommission
 freezes the Namenode
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    [ https://issues.apache.org/jira/browse/HADOOP-4061?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649821#action_12649821 ] 

Konstantin Shvachko commented on HADOOP-4061:
---------------------------------------------

This should work, but may be it would be even better not create any references at all.
I propose to store last checked {{DatanodeDescriptor lastDN}} in DatanodeManager and get a {{datanodeMap.tail(lastDN, nonInclusive)}} on each iteration. The tail map can be used instead of the queue, since it has analogous methods {{getFirstEntry()}} and {{pollFirstEntry()}}.

> Large number of decommission freezes the Namenode
> -------------------------------------------------
>
>                 Key: HADOOP-4061
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4061
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.17.2
>            Reporter: Koji Noguchi
>            Assignee: Tsz Wo (Nicholas), SZE
>         Attachments: 4061_20081119.patch, 4061_20081120.patch, 4061_20081120b.patch
>
>
> On 1900 nodes cluster, we tried decommissioning 400 nodes with 30k blocks each. Other 1500 nodes were almost empty.
> When decommission started, namenode's queue overflowed every 6 minutes.
> Looking at the cpu usage,  it showed that every 5 minutes org.apache.hadoop.dfs.FSNamesystem$DecommissionedMonitor thread was taking 100% of the CPU for 1 minute causing the queue to overflow.
> {noformat}
>   public synchronized void decommissionedDatanodeCheck() {
>     for (Iterator<DatanodeDescriptor> it = datanodeMap.values().iterator();
>          it.hasNext();) {
>       DatanodeDescriptor node = it.next();
>       checkDecommissionStateInternal(node);
>     }
>   }
> {noformat}

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