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https://issues.apache.org/jira/browse/HDFS-3889?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13450066#comment-13450066
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Colin Patrick McCabe commented on HDFS-3889:
--------------------------------------------
bq. If the goal is to just provide the same functionality as rsync, then sure.
Although I consider those less reliable (or just as bad) as file size alone.
They require the metadata to be kept in sync between source and destination,
something that I don't think is very common for mod time or access time, for
example.
I believe that the modification time is set based on the NN, not the clients.
So nothing needs to be kept in sync. It's true that time can sometimes go
backwards on the NN (due to server misconfiguration, NTP, or other things) but
it's not exactly common.
Still, I could go either way on this point. It's nice to know that you're
doing the safe thing, and refusing to skip pre-copy checksum definitely is the
safe thing.
Also, we currently aren't doing as much checking as we should do. We don't
consider the mtime and owner, group, etc at the moment. This makes skipping
the checksum a lot more unsafe than it needs to be.
> 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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