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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 20:20:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12534146
] 

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


Without appends, a block and .meta file are written to only once. With this, .meta will be
written to many times. Ignoring appends, probability of write corruption  for a given byte
around the 'last verification time' would be orders of magnitude *more* than other data on
the disk and not less.

I will increase the .meta version and write this information at the the beginning of the file.
I am still not very sure of benifits of persisting the verification time.. but there might
be more.


> 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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