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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 19:19:51 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12534131
] 

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

> Why not keep the last verification time in each blocks .meta file or use the native filesystem
timestamp for this purpose?

Many disk errors occur during the write path. If we modify .meta file that contains .crc every
few days, we might actually increase error rate.

Also writing to metafile will be at least one random seek/write for each verification.

If above two are ok, we can put this info in .meta file. I viewed last verification time as
non-critical data, that does not need to be maintained persistently.


> 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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