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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Tue, 27 Mar 2007 18:57:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484545
] 

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


>> 64KB is only for new blocks.
> I thought we'd benchmark before we changed any parameters, no? I'd vote to keep this
at 512 until we benchmark.

I am fine with keeping it at 512. default config will not change then. Bigger motivation for
me to run good benchmarks :-)

> This sounds like another change we should benchmark before we make.

I think this is as much a policy decision as it is a performance decision. Do we ever want
datanode to ship corrupted data? If the benchmarks don't show any negative effect of double
checksum on real clusters, would that be enough?

If source datanode does not check for errors, how would you propose we handle CRC errors while
transferring to another datanode? I guess, destination datanode should report the problem.



> 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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