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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7648) Verify the datanode directory layout
Date Fri, 30 Jan 2015 23:46:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7648?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14299387#comment-14299387
] 

Colin Patrick McCabe commented on HDFS-7648:
--------------------------------------------

bq. The original design of DirectoryScanner is to reconciles the differences between the block
information maintained in memory and the actual blocks stored in disks. So it does fix the
in-memory data structure.

Fixing the in-memory data structure is different than fixing the on-disk data structure. 
I do not think that the DirectoryScanner should modify the files on the disk.  It just introduces
too much potential for error and mistakes in the scanner to cause data loss.

bq. Yet more questions if the blocks are not fixed: should the block report include those
blocks? How to access those blocks? How and when to fix those blocks?

The only way we could ever get into this state is:
* if someone manually renamed some block files on ext4
* if someone introduced a bug in the datanode code that put blocks in the wrong place.
* if there is serious ext4 filesystem corruption

None of those cases seems like something we should be trying to automatically recover from.

> Verify the datanode directory layout
> ------------------------------------
>
>                 Key: HDFS-7648
>                 URL: https://issues.apache.org/jira/browse/HDFS-7648
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Tsz Wo Nicholas Sze
>            Assignee: Rakesh R
>
> HDFS-6482 changed datanode layout to use block ID to determine the directory to store
the block.  We should have some mechanism to verify it.  Either DirectoryScanner or block
report generation could do the check.



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