[jira] Issue Comment Edited: (HADOOP-2012) Periodic verification at the Datanode

["Raghu Angadi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-2012?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12538891 ] 

rangadi edited comment on HADOOP-2012 at 10/30/07 12:20 PM:
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> The metadata about the entire Datanode is stored in the VERSION file. It is possible that we can store the last verified-blockid in this file (instead of adding a new file).

VERSION file contains only the basic information needed for starting up a Datanode and is vital for Datanode startup. It is not updated at runtime. I don't think it is suited for for this.

Regd continuous scanning, I think most users would not prefer that. Even 5MB per sec is close to 20% of a single disk read and much more if the read/write is not very sequential (happens when there are multiple reads and writes).

We can certainly make the SCAN_PERIOD and throttle bandwidth configurable (may be not in hadoop-defaults.xml), where power users can tweak it as appropriate. I know there is strong resistance for adding any config variables :). But my personal opinion is that some more config vars that 99% of users don't need to worry about because of good defaults are ok. 


      was (Author: rangadi):
    > The metadata about the entire Datanode is stored in the VERSION file. It is possible that we can store the last verified-blockid in this file (instead of adding a new file).

VERSION file contains only the basic information needed for starting up a Datanode and is vital for Datanode startup. It is not updated at runtime. I don't think it is suited for for this.

Regd continuous scanning, I think most users would not prefer that. Even 5MB per sec is close to 20% of a single disk read and much more if the read/write is not very sequential (happens when there are multiple reads and writes).

We can certainly make the SCAN_PERIOD and throttle bandwidth configurable (may be not in hadoop-defaults.xml), where power users can tweak it as appropriate. I know there is strong resistance for adding any config variables :). But I personal opinion is that some more config's that 99% of users don't need to worry about because of good defaults are ok. 

  
> 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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