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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482917
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Doug Cutting commented on HADOOP-1134:
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> CRC-chunk should be configurable for new blocks and client does not need to
> know a priory what the chunk size is.
I'm not sure what "CRC-chunk" is, but I see no reason to add new config
parameters right off. We can continue to use io.bytes.per.checksum and perhaps
throw an exception if dfs.block.size is not an even multiple of this. If it
later makes sense to increase the default bytes/checksum for DFS, it may also
make sense to increase the default for all other filesystems too, continuing to
use the same parameter, or it may make sense to split this to a new parameter,
but, for now, I see no reason to add a new parameter.
> 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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