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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 Fri, 12 Sep 2014 22:24:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14132214#comment-14132214
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Jitendra Nath Pandey commented on HDFS-6978:
--------------------------------------------

+1 for the patch.

A minor comment:
{code}
+          if (d < blockpoolReport.length) {
+            // There may be multiple on-disk records for the same block, don't increment
+            // the memory record pointer if so. 
+            ScanInfo nextInfo = blockpoolReport[Math.min(d, blockpoolReport.length - 1)];
{code}
Math.min(d, blockpoolReport.length - 1) will always be equal to d, unless we have race conditions.
I think the same applies to the previous lines with Math.min as well.


> 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
>
>
> 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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