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Mithun Radhakrishnan commented on MAPREDUCE-5065: ------------------------------------------------- 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. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira