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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8486) DN startup may cause severe data loss
Date Thu, 28 May 2015 16:22:17 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14563196#comment-14563196
] 

Daryn Sharp commented on HDFS-8486:
-----------------------------------

What you'll notice is a spike in corrupt blocks that tapers down.  What's going on is the
DN's block report included all the blocks it deleted.  Over the next 6 hours, the slice scanner
slowly detects missing blocks and reports them as corrupt.  After 6 hours, the directory scanner
detects and mass removes all the missing blocks.

In the 6 hour window, the NN does not know the block is under-replicated and it continues
to send clients to the DN.  Will file a separate bug for the DN not informing the NN when
it's missing a block it thought it had.

> DN startup may cause severe data loss
> -------------------------------------
>
>                 Key: HDFS-8486
>                 URL: https://issues.apache.org/jira/browse/HDFS-8486
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>    Affects Versions: 0.23.1, 2.0.0-alpha
>            Reporter: Daryn Sharp
>            Assignee: Daryn Sharp
>            Priority: Blocker
>
> A race condition between block pool initialization and the directory scanner may cause
a mass deletion of blocks in multiple storages.
> If block pool initialization finds a block on disk that is already in the replica map,
it deletes one of the blocks based on size, GS, etc.  Unfortunately it _always_ deletes one
of the blocks even if identical, thus the replica map _must_ be empty when the pool is initialized.
> The directory scanner starts at a random time within its periodic interval (default 6h).
 If the scanner starts very early it races to populate the replica map, causing the block
pool init to erroneously delete blocks.



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