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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-767) Job failure due to BlockMissingException
Date Fri, 13 Nov 2009 07:38:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12777414#action_12777414

Ning Zhang commented on HDFS-767:

Todd, you are right if we want to establish the time windows at the point of the first request.
And your calculated time is the total wait time. My formula consider the next wait time which
is 0..3 for the first attempt, 3..9 for 2nd attempt and 6..15 for 3rd attempt. The total wait
time range is 0..3, 3..12, and 9..27 for the first 3 attempts. 

One reason for my formula is its simplicity, we don't need to keep track of the first trial
time and current time. It only depends on 1 parameter (the # of failures). The second reason
is that we don't know how much time will be spent from sending out the request to getting
a failure. it could be tens to hundreds of milliseconds due to network roundtrip and NameNode
workload. We don't want to add that into the consideration. 

> 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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