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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4116) Balancer should provide better resource management
Date Wed, 24 Sep 2008 09:27:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4116?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12634074#action_12634074
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Steve Loughran commented on HADOOP-4116:
----------------------------------------

keepAlives are a fairly weak way of assessing "liveness" because 
 -it works at the network stack level, so your app may still be dead but the KA packets are
happy
 -if there are a lot of (idle) connections between two hosts, a lot of KA traffic can be generated,
rather than one packet per host, which is how a lot of protocols (CORBA and DCOM, for example)
communicate "we are still alive". 

I think this proposal is better than nothing, but we need to be aware of limitations. It will
detect a network partition, but not a hung far end if the network stack is still up

> Balancer should provide better resource management
> --------------------------------------------------
>
>                 Key: HADOOP-4116
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4116
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.17.0
>            Reporter: Raghu Angadi
>            Assignee: Hairong Kuang
>            Priority: Blocker
>             Fix For: 0.18.2, 0.19.0
>
>         Attachments: balancerRM.patch, balancerRM1.patch
>
>
> The number of threads are currently limited on datanodes. Once these threads are occupied,
DataNode does not accept any more requests (DOS). Recently we saw a case where most of the
256 threads were waiting in {{DataXceiver.replaceBlock()}} trying to acquire  {{balancingSem}}.
 Since rebalancing  is (heavily) throttled, I would think this would be the common case. 
> These operations waiting  for active rebalancing threads to finish need not take up a
thread. 

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