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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6772) Get DNs out of blockContentsStale==true state faster when NN restarts
Date Thu, 07 Aug 2014 19:21:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14089650#comment-14089650
] 

Arpit Agarwal commented on HDFS-6772:
-------------------------------------

Also the test case should be wrapped in try.. finally so it can shutdown the cluster if the
test fails midway.

> Get DNs out of blockContentsStale==true state faster when NN restarts
> ---------------------------------------------------------------------
>
>                 Key: HDFS-6772
>                 URL: https://issues.apache.org/jira/browse/HDFS-6772
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ming Ma
>            Assignee: Ming Ma
>         Attachments: HDFS-6772.patch
>
>
> Here is the non-HA scenario.
> 1. Get HDFS into block-over-replicated situation.
> 2. Restart the NN.
> 3. From NN's point of view, DNs will remain in blockContentsStale==true state for a long
time. That in turns make postponedMisreplicatedBlocks size big. Bigger postponedMisreplicatedBlocks
size will impact blockreport latency. Given blockreport takes NN global lock, it has severe
impact on NN performance and make the cluster unstable.
> Why will DNs remain in blockContentsStale==true state for a long time?
> 1. When a DN reconnect to NN upon NN restart, blockreport RPC could come in before heartbeat
RPC. That is due to how BPServiceActor#offerService decides when to send blockreport and heartbeat.
In the case of NN restart, NN will ask DN to register when NN gets the first heartbeat request;
DN will then register with NN; followed by blockreport RPC; the heartbeat RPC will come after
that.
> 2. So right after the first blockreport, given heartbeatedSinceFailover remains false,
blockContentsStale will stay true.
> {noformat}
> DatanodeStorageInfo.java
>   void receivedBlockReport() {
>     if (heartbeatedSinceFailover) {
>       blockContentsStale = false;
>     }
>     blockReportCount++;
>   }
> {noformat}
> 3. So the DN will remain in blockContentsStale==true until the next blockreport. For
big cluster, dfs.blockreport.intervalMsec could be set to some large value.
>  



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