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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 22 Mar 2007 21:13:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12483325
] 

Raghu Angadi commented on HADOOP-1134:
--------------------------------------


I think thinner and more detached the client is, better in long term. DFSClient is not part
of DFS cluster, because it is not expected to be up and running when cluster is running. Tighter
the coupling, more difficult it will be to implement new features in future. 

I don't think the issue is just about where the checksums are stored. I interpreted as: DFS
(excluding the DFSClient) will be aware of and in complete control of how data is stored and
its integrity is maintained. To me it does not sound right if DFS can't change how its data
is maintained in future. Of course DFS still needs to be able to serve data it is supposed
to serve.. thats its fundamental duty. 

Just my thoughts.. will ask more issue specific questions soon.


> Block level CRCs in HDFS
> ------------------------
>
>                 Key: HADOOP-1134
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1134
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: Raghu Angadi
>         Assigned To: Raghu Angadi
>
> Currently CRCs are handled at FileSystem level and are transparent to core HDFS. See
recent improvement HADOOP-928 ( that can add checksums to a given filesystem ) regd more about
it. Though this served us well there a few disadvantages :
> 1) This doubles namespace in HDFS ( or other filesystem implementations ). In many cases,
it nearly doubles the number of blocks. Taking namenode out of CRCs would nearly double namespace
performance both in terms of CPU and memory.
> 2) Since CRCs are transparent to HDFS, it can not actively detect corrupted blocks. With
block level CRCs, Datanode can periodically verify the checksums and report corruptions to
namnode such that name replicas can be created.
> We propose to have CRCs maintained for all HDFS data in much the same way as in GFS.
I will update the jira with detailed requirements and design. This will include same guarantees
provided by current implementation and will include a upgrade of current data.
>  

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