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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5074) Allow starting up from an fsimage checkpoint in the middle of a segment
Date Sun, 11 Aug 2013 00:44:47 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5074?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13736144#comment-13736144
] 

Todd Lipcon commented on HDFS-5074:
-----------------------------------

Yes, the Tailer will retry and will read from a different JN the next time around. When it
does so, it properly "fast-forwards" the stream to the right transaction. But, if a checkpoint
was overdue, it may make a checkpoint before it gets a chance to retry, which can provoke
this issue on the next restart of the SBN if it doesn't make an "aligned" checkpoint before
then.
                
> Allow starting up from an fsimage checkpoint in the middle of a segment
> -----------------------------------------------------------------------
>
>                 Key: HDFS-5074
>                 URL: https://issues.apache.org/jira/browse/HDFS-5074
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: ha, namenode
>    Affects Versions: 3.0.0, 2.1.0-beta
>            Reporter: Todd Lipcon
>
> We've seen the following behavior a couple times:
> - SBN is running and somehow encounters an error in the middle of replaying an edit log
in the tailer (eg the JN it's reading from crashes)
> - SBN successfully has processed half of the edits in the segment it was reading.
> - SBN saves a checkpoint, which now falls in the middle of a segment, and then restarts
> Upon restart, the SBN will load this checkpoint which falls in the middle of a segment.
{{selectInputStreams}} then fails when the SBN requests a mid-segment txid.
> We should handle this case by downloading the right segment and fast-forwarding to the
correct txid.

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