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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482861
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Doug Cutting commented on HADOOP-1134:
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> Do you mean upgrade would not actually read the blocks if we are using old
> CRC?
Yes. And, as I noted above, strictly speaking, this is required for
correctness. We cannot simply checksum blocks, assuming they're not corrupt.
The existing checksum data was created on the client and is much more
trustworthy. So if we're going to re-compute checksums we should first
validate the data against the old checksums. But it might be easier to simply
re-use the existing checksums.
I'm all for moving to something like 64k bytes/checksum for new files (and old
files, if they're validated), although we ought to benchmark the cost of
transferring and checksumming 64k before we do this as that will be added to
the cost of a seek. We should test that seek performance does not
significantly suffer. Note that the entire 64k chunk must be transferred to
the client for checksumming, so the added cost per seek is not just computation
and disk time, but network bandwidth too.
> 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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