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From "Lin Yiqun (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-13091) DistCp masks potential CRC check failures
Date Thu, 05 May 2016 06:47:13 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-13091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15271976#comment-15271976

Lin Yiqun commented on HADOOP-13091:

Thanks for everyone's comments. All the comments looks good to me.
Hi, [~cnauroth], your proposol is actually better than dependenting on current exception handling
logic. But the current exception handling logic can also test all the special cases in {{DistCpUtils.checksumsAreEqual}}.
Can we use the existing unit tests and then do the optimization? Also I'm happy to do further
optimization in the unit test now if we want to make this better.
Update the v003 patch for addressing the commets. Pending jenkins.

> DistCp masks potential CRC check failures
> -----------------------------------------
>                 Key: HADOOP-13091
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13091
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 2.7.1
>            Reporter: Elliot West
>            Assignee: Lin Yiqun
>         Attachments: HDFS-10338.001.patch, HDFS-10338.002.patch
> There appear to be edge cases whereby CRC checks may be circumvented when requests for
checksums from the source or target file system fail. In this event CRCs could differ between
the source and target and yet the DistCp copy would succeed, even when the 'skip CRC check'
option is not being used.
> The code in question is contained in the method [{{org.apache.hadoop.tools.util.DistCpUtils#checksumsAreEqual(...)}}|https://github.com/apache/hadoop/blob/release-2.7.1/hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/util/DistCpUtils.java#L457]
> Specifically this code block suggests that if there is a failure when trying to read
the source or target checksum then the method will return {{true}} (i.e.  the checksums are
equal), implying that the check succeeded. In actual fact we just failed to obtain the checksum
and could not perform the check.
> {code}
> try {
>   sourceChecksum = sourceChecksum != null ? sourceChecksum : 
>     sourceFS.getFileChecksum(source);
>   targetChecksum = targetFS.getFileChecksum(target);
> } catch (IOException e) {
>   LOG.error("Unable to retrieve checksum for " + source + " or "
>     + target, e);
> }
> return (sourceChecksum == null || targetChecksum == null ||
>   sourceChecksum.equals(targetChecksum));
> {code}
> I believe that at the very least the caught {{IOException}} should be re-thrown. If this
is not deemed desirable then I believe an option ({{--strictCrc}}?) should be added to enforce
a strict check where we require that both the source and target CRCs are retrieved, are not
null, and are then compared for equality. If for any reason either of the CRCs retrievals
fail then an exception is thrown.
> Clearly some {{FileSystems}} do not support CRCs and invocations to {{FileSystem.getFileChecksum(...)}}
return {{null}} in these instances. I would suggest that these should fail a strict CRC check
to prevent users developing a false sense of security in their copy pipeline.

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