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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2012) Periodic verification at the Datanode
Date Thu, 11 Oct 2007 19:50:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12534140
] 

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

> Would writing more to the .meta file increase the chance of the .meta file being corrupted?
I'm not sure I see that.

Even if we are writing 8 bytes, if there a problem during the write (hardware / driver error),
I would usually corrupt a few sectors instead of just a few bytes. More times we write to
the same location of a file more the chances there is an error.

I might be wrong, may be errors that occur during writes is much much less than other ways
disks go corrupt.. but I have seen quite a few kernel / driver errors while writing.


> 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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