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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Fri, 23 Mar 2007 00:33:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12483367
] 

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

Thank Doug.

>> How the data is checksummed at the client while reading could be different from how
the data is checksummed while writing.
> I'd vote against that for this issue. 

Do you agree with the policy in principle? We may not do that as part of this issue.

How the over all interaction will look like in future will affect how we organize our code
for this issue even if we limit the changes. I will list various things we want to or intend
to do and we can pick the required things as part of this issue.

>> If some checksum does not match with two others, then it is considered corrupt.
> That would not catch corruptions that happened before the first copy is received. I'm
not fully comfortable with this shortcut.

True. I will see how bad getting old CRCs would be...

> 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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