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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-767) Job failure due to BlockMissingException
Date Fri, 13 Nov 2009 20:18:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12777635#action_12777635
] 

Todd Lipcon commented on HDFS-767:
----------------------------------

Hi Ning,

Sounds good - your formula seems to make sense. If you can add a few lines of comments around
the formula (or a pointer to this JIRA) I think that would be helpful to make sure people
looking at the code down the line will understand where it came from.

Additionally, I think making the 3000 parameter a configuration variable (even if an undocumented
one) would be swell.

> 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
>
> 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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