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