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From "Marcelo Vanzin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3889) distcp overwrites files even when there are missing checksums
Date Thu, 06 Sep 2012 21:40:10 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3889?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13450075#comment-13450075
] 

Marcelo Vanzin commented on HDFS-3889:
--------------------------------------

bq. I believe that the modification time is set based on the NN, not the clients. So nothing
needs to be kept in sync.

You have two NNs. The metadata on the the target NN needs to be in sync with the source NN
for the metadata-based check to do the right thing.

In the end, my opinion is just that metadata-based checks are a very poor substitute for checksums,
and can much more easily generate false positives (i.e. say that files are equal when they're
not). But if it's a feature that people find useful, why not. The false negative case is not
such a big problem, since it would just waste bandwidth by forcing the copy.
                
> distcp overwrites files even when there are missing checksums
> -------------------------------------------------------------
>
>                 Key: HDFS-3889
>                 URL: https://issues.apache.org/jira/browse/HDFS-3889
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: tools
>    Affects Versions: 2.2.0-alpha
>            Reporter: Colin Patrick McCabe
>            Priority: Minor
>
> If distcp can't read the checksum files for the source and destination files-- for any
reason-- it ignores the checksums and overwrites the destination file.  It does produce a
log message, but I think the correct behavior would be to throw an error and stop the distcp.
> If the user really wants to ignore checksums, he or she can use {{-skipcrccheck}} to
do so.
> The relevant code is in DistCpUtils#checksumsAreEquals:
> {code}
>     try {
>       sourceChecksum = sourceFS.getFileChecksum(source);
>       targetChecksum = targetFS.getFileChecksum(target);
>     } catch (IOException e) {
>       LOG.error("Unable to retrieve checksum for " + source + " or " + target, e);
>     }
> {code}

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