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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2012) Periodic verification at the Datanode
Date Wed, 17 Oct 2007 20:04:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12535712
] 

Raghu Angadi commented on HADOOP-2012:
--------------------------------------

> From what I know it's the Namenode, which I believe doesn't do anything with the reports
other than logging the failure.

Namenode actually deletes the block if there are more than one copy. Periodic verification
uses that same RPC.

> It would be better to report the failure to the Datanode, have the Datanode validate
what's on disk and report corruption to the Namenode if the validation fails.

I agree. This will avoid spurious block deletion during in memory corruption.


> Periodic verification at the Datanode
> -------------------------------------
>
>                 Key: HADOOP-2012
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2012
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>
> Currently on-disk data corruption on data blocks is detected only when it is read by
the client or by another datanode.  These errors are detected much earlier if datanode can
periodically verify the data checksums for the local blocks.
> Some of the issues to consider :
> - How should we check the blocks ( no more often than once every couple of weeks ?)
> - How do we keep track of when a block was last verfied ( there is a .meta file associcated
with each lock ).
> - What action to take once a corruption is detected
> - Scanning should be done as a very low priority with rest of the datanode disk traffic
in mind.

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