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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-767) Job failure due to BlockMissingException
Date Tue, 01 Dec 2009 20:15:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784390#action_12784390
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Raghu Angadi commented on HDFS-767:
-----------------------------------

I just briefly looked at it. 

Essentially, you are randomizing the retry times without actually increasing the number of
retries (retry interval is increased). In that case, we will still see failures if the fetches
take longer than a few seconds (a few seconds is quite possible, if you have a lot of threads
reading from the disk, each client will take longer to read same amount of data).

+1 for the patch, as a work around for some situations.

> 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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