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https://issues.apache.org/jira/browse/MAPREDUCE-5065?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13603418#comment-13603418
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Mithun Radhakrishnan commented on MAPREDUCE-5065:
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I'm with you on the need for a blocksize-independent checksum. I wasn't
convinced that combining CRC32-checksums together to form a higher-level
checksum could be correct. (Thanks for the explanation.)
{quote}
instruct her to run with -pb, not -skipCrc.
{quote}
Yep, that should take care of #2 (above), but not #1. The user will still need
to fail first and rerun, because she's unlikely to know that some of her
source-files might have non-default block-sizes. Unless the checksum
calculation is fixed (or -pb is default), I don't think DistCp should enforce a
check that's a guaranteed failure, under unforeseeable circumstances.
> DistCp should skip checksum comparisons if block-sizes are different on
> source/target.
> --------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-5065
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5065
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: distcp
> Affects Versions: 2.0.3-alpha, 0.23.5
> Reporter: Mithun Radhakrishnan
> Assignee: Mithun Radhakrishnan
> Attachments: MAPREDUCE-5065.branch23.patch,
> MAPREDUCE-5065.branch2.patch
>
>
> When copying files between 2 clusters with different default block-sizes, one
> sees that the copy fails with a checksum-mismatch, even though the files have
> identical contents.
> The reason is that on HDFS, a file's checksum is unfortunately a function of
> the block-size of the file. So you could have 2 different files with
> identical contents (but different block-sizes) have different checksums.
> (Thus, it's also possible for DistCp to fail to copy files on the same
> file-system, if the source-file's block-size differs from HDFS default, and
> -pb isn't used.)
> I propose that we skip checksum comparisons under the following conditions:
> 1. -skipCrc is specified.
> 2. File-size is 0 (in which case the call to the checksum-servlet is moot).
> 3. source.getBlockSize() != target.getBlockSize(), since the checksums are
> guaranteed to differ in this case.
> I have a patch for #3.
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