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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485292
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Doug Cutting commented on HADOOP-1134:
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> Even if we *can* get the old CRC data, how do we know that it is not corrupt?
The same way we do today: we don't. If checksums are corrupt, we assume the
data is corrupt, which is right 99% of the time, since the data is 100 times
larger. I don't think we should try to improve the checksum quality for
existing data with this patch. (Who will checksum the checksums?) That's
mission creep. It's a laudable long-term goal. This patch should confine
itself to removing checksums from the HDFS namespace. That will be difficult
enough to develop, debug, review, test, deploy and support. We should not use
this issue for a complete re-design of checksumming. Rather we can
incrementally optimize and improve the checksum system after this issue is
committed.
> 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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