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Date: Sun, 13 Dec 2009 19:38:18 +0000 (UTC)
From: "He Yongqiang (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-767) Job failure due to
 BlockMissingException
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    [ https://issues.apache.org/jira/browse/HDFS-767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12789933#action_12789933 ] 

He Yongqiang commented on HDFS-767:
-----------------------------------

+1

> Job failure due to BlockMissingException
> ----------------------------------------
>
>                 Key: HDFS-767
>                 URL: https://issues.apache.org/jira/browse/HDFS-767
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ning Zhang
>            Assignee: Ning Zhang
>         Attachments: HDFS-767.patch
>
>
> If a block is request by too many mappers/reducers (say, 3000) at the same time, a BlockMissingException is thrown because it exceeds the upper limit (I think 256 by default) of number of threads accessing the same block at the same time. The DFSClient wil catch that exception and retry 3 times after waiting for 3 seconds. Since the wait time is a fixed value, a lot of clients will retry at about the same time and a large portion of them get another failure. After 3 retries, there are about 256*4 = 1024 clients got the block. If the number of clients are more than that, the job will fail. 

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