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From "Jitendra Nath Pandey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6978) Directory scanner should correctly reconcile blocks on RAM disk
Date Sat, 13 Sep 2014 05:05:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14132544#comment-14132544
] 

Jitendra Nath Pandey commented on HDFS-6978:
--------------------------------------------

+1

> Directory scanner should correctly reconcile blocks on RAM disk
> ---------------------------------------------------------------
>
>                 Key: HDFS-6978
>                 URL: https://issues.apache.org/jira/browse/HDFS-6978
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: datanode
>    Affects Versions: HDFS-6581
>            Reporter: Arpit Agarwal
>            Assignee: Arpit Agarwal
>         Attachments: HDFS-6978.01.patch, HDFS-6978.02.patch
>
>
> It used to be very unlikely that the directory scanner encountered two replicas of the
same block on different volumes.
> With memory storage, it is very likely to hit this with the following sequence of events:
> # Block is written to RAM disk
> # Lazy writer saves a copy on persistent volume
> # DN attempts to evict the original replica from RAM disk, file deletion fails as the
replica is in use.
> # Directory scanner finds a replica on both RAM disk and persistent storage.
> The directory scanner should never delete the block on persistent storage.



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