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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12486411
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Doug Cutting commented on HADOOP-1134:
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> 3) If there is no existing CRC data available for some reason, we generate
> from existing block data. A warning will be printed.
I'm not sure about this one. Generating a checksum from existing data gives
the appearance that the data is correct when it might not be, which could be
dangerous. I think we should handle this the same way we'll handle things
if/when a CRC file is missing after the upgrade. That shouldn't happen, but it
might, and we need to think about what we should do in that case. My guess is
that we should return a null checksum with the data when it is read, and let
the client decide whether to accept or reject the unchecksummed data.
Currently I think we emit a warning and accept it.
> 4.b) If can not find an upgraded replica, it generates local CRC.. Or should
> it delete it?
This should be handled as above. We should not generate a new CRC, but rather
leave the block without a CRC file and pass that information along to the
client at read time. The client can then decide to look for a replica with a
CRC, emit a warning, fail, etc.
> 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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