[jira] [Comment Edited] (HADOOP-16083) DistCp shouldn't always overwrite the target file when checksums match

["Siyao Meng (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-16083?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16755424#comment-16755424 ] 

Siyao Meng edited comment on HADOOP-16083 at 1/29/19 10:15 PM:
---------------------------------------------------------------

[~stevel@apache.org] Thanks for the comment.
The case involved here is *single file* copy with CRC check enabled. If the target path is a directory, *for HDFS* it will skip files that has the same checksum. For example, the command below will always copy and overwrite the file at the target.
{code:bash|title=Target path is a file}
$ hadoop distcp -update hdfs:///src/2.txt hdfs:///dst/2.txt
...
		Bytes Copied=6
		Bytes Expected=6
		Files Copied=1
{code}
But this command wouldn't skip the copy if the target file exists and is the same, note the target path is a directory now instead of a file.
{code:bash|title=Target path is a directory}
$ hadoop distcp -update hdfs:///src/2.txt hdfs:///dst/
...
		Bytes Skipped=6
		Files Skipped=1
{code}

I'm aware that the change might lead to subtle failures of other applications. For HDFS, so far I figured the only thing that changed with the patch would be the modification time of the file on target FS. This is tested in TestCopyMapper#testSingleFileCopy() and TestCopyMapperCompositeCrc#testSingleFileCopy(). Both test case failures would be addressed in rev 002.

I agree that if the target FS 1. doesn't support checksum; or 2. disabled checksum; or 3. doesn't support timestamp modification (touch), the file should always be copied.

What new tests should be added in AbstractContractDistCpTest?


was (Author: smeng):
[~stevel@apache.org] Thanks for the comment.
The case involved here is *single file* copy with CRC check enabled. If the target path is a directory, *for HDFS* it will skip files that has the same checksum. For example, the command below will always copy and overwrite the file at the target.
{code:bash|title=Target path is a file}
$ hadoop distcp -update hdfs:///src/2.txt hdfs:///dst/2.txt
...
		Bytes Copied=6
		Bytes Expected=6
		Files Copied=1
{code}
But this command wouldn't skip the copy is the target file exists and is the same, note the target path is a directory now instead of a file.
{code:bash|title=Target path is a directory}
$ hadoop distcp -update hdfs:///src/2.txt hdfs:///dst/
...
		Bytes Skipped=6
		Files Skipped=1
{code}

I'm aware that the change might lead to subtle failures of other applications. For HDFS, so far I figured the only thing that changed with the patch would be the modification time of the file on target FS. This is tested in TestCopyMapper#testSingleFileCopy() and TestCopyMapperCompositeCrc#testSingleFileCopy(). Both test case failures would be addressed in rev 002.

I agree that if the target FS 1. doesn't support checksum; or 2. disabled checksum; or 3. doesn't support timestamp modification (touch), the file should always be copied.

What new tests should be added in AbstractContractDistCpTest?

> DistCp shouldn't always overwrite the target file when checksums match
> ----------------------------------------------------------------------
>
>                 Key: HADOOP-16083
>                 URL: https://issues.apache.org/jira/browse/HADOOP-16083
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: tools/distcp
>    Affects Versions: 3.2.0, 3.1.1, 3.3.0
>            Reporter: Siyao Meng
>            Assignee: Siyao Meng
>            Priority: Major
>         Attachments: HADOOP-16083.001.patch
>
>
> {code:java|title=CopyMapper#setup}
> ...
>     try {
>       overWrite = overWrite || targetFS.getFileStatus(targetFinalPath).isFile();
>     } catch (FileNotFoundException ignored) {
>     }
> ...
> {code}
> The above code overrides config key "overWrite" to "true" when the target path is a file. Therefore, unnecessary transfer happens when the source and target file have the same checksums.
> My suggestion is: remove the code above. If the user insists to overwrite, just add -overwrite in the options:
> {code:bash|title=DistCp command with -overwrite option}
> hadoop distcp -overwrite hdfs://localhost:64464/source/5/6.txt hdfs://localhost:64464/target/5/6.txt
> {code}



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