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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2012) Periodic verification at the Datanode
Date Thu, 25 Oct 2007 18:14:51 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12537676
] 

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

> 2) It seems to me it might be better to try to repair the block if possible, rather then
just delete it. This avoids bad corner cases. It adds complexity though. Thoughts? A simple
variant is just to copy a new version locally.

Datanode actually does not physically delete the block after detecting corruption. It asks
the Namenode to delete the block (just like a client does when it detects corruption). Namenode
deletes the blocks only if there are more replicas left and then replicates the block. Does
this address the case?

Regd (1)  and Dhruba's comment, a log file would be my preferred approach too. But main question
I get asked is "Why add another file?".

will think about (3).



> 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
>             Fix For: 0.16.0
>
>         Attachments: HADOOP-2012.patch, HADOOP-2012.patch, HADOOP-2012.patch, HADOOP-2012.patch
>
>
> 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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