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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484538
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Doug Cutting commented on HADOOP-1134:
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> 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 personally prefer datanode verification (and general ownership of CRCs). I
> don't think it adds noticeable latency.
This sounds like another change we should benchmark before we make. This patch
should confine itself to a single issue: removing the checksum data from the
DFS namespace. Subsequently we should determine the performance impacts of
double-checksumming (on both datanode & client), larger checksum windows, etc.,
before committing such changes.
Changing the storage of checksum data should alone improve performance, since
fewer datanode and namenode accesses are required. It will be useful to see
how much this change alone improves things before we start making other
optimizations.
> 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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