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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11708) positional read will fail if replicas moved to different DNs after stream is opened
Date Tue, 23 May 2017 01:42:05 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11708?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16020545#comment-16020545
] 

Konstantin Shvachko commented on HDFS-11708:
--------------------------------------------

Not sure I understand what you are trying to fix here. It looks like that {{chooseDataNode()}}
already calls {{refreshLocatedBlock()}} in case when all other locations failed. So your patch
adds another call to NameNode unconditionally on retry, probably not the best thing to do
from performance viewpoint.
It would be good if you could provide a unit test, that fails without your fix.

> positional read will fail if replicas moved to different DNs after stream is opened
> -----------------------------------------------------------------------------------
>
>                 Key: HDFS-11708
>                 URL: https://issues.apache.org/jira/browse/HDFS-11708
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs-client
>    Affects Versions: 2.7.3
>            Reporter: Vinayakumar B
>            Assignee: Vinayakumar B
>            Priority: Critical
>              Labels: release-blocker
>         Attachments: HDFS-11708-01.patch, HDFS-11708-02.patch, HDFS-11708-03.patch, HDFS-11708-04.patch
>
>
> Scenario:
> 1. File was written to DN1, DN2 with RF=2
> 2. File stream opened to read and kept. Block Locations are [DN1,DN2]
> 3. One of the replica (DN2) moved to another datanode (DN3) due to datanode dead/balancing/etc.
> 4. Latest block locations in NameNode will be DN1 and DN3 in the 'same order'
> 5. DN1 went down, but not yet detected as dead in NameNode.
> 6. Client start reading using positional read api "read(pos, buf[], offset, length)"



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