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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482883
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Doug Cutting commented on HADOOP-1134:
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> Why not send only 2k and send newly calculated CRC for 2k?
Perhaps that could work. So we'd still need to transfer 64k off disk and
checksum it, but, once it's validated, we could re-checksum the 2k we send. It
should be re-checksummed before it's validated, so that the re-checksummed data
is guaranteed valid. On the other hand, the simplicity of the end-to-end
checksum makes it more certain that we've implemented things correctly and will
properly detect corruptions. +0
> How do we benchmark for good CRC-chunk size?
Can we push that to a separate issue? This issue is about removing checksum
files from the HDFS namespace. We might then optimize things in other ways
later.
> 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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