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https://issues.apache.org/jira/browse/HADOOP-18596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17678768#comment-17678768
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ASF GitHub Bot commented on HADOOP-18596:
-----------------------------------------
dannycjones commented on code in PR #5308:
URL: https://github.com/apache/hadoop/pull/5308#discussion_r1081452359
##########
hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/mapred/CopyMapper.java:
##########
@@ -354,7 +354,14 @@ private boolean canSkip(FileSystem sourceFS,
CopyListingFileStatus source,
boolean sameLength = target.getLen() == source.getLen();
boolean sameBlockSize = source.getBlockSize() == target.getBlockSize()
|| !preserve.contains(FileAttribute.BLOCKSIZE);
- if (sameLength && sameBlockSize) {
+ // checksum check to be done if same file len(greater than 0), same block
+ // size and the target file has been updated more recently than the source
+ // file.
+ // Note: For Different cloud stores with different checksum algorithms,
+ // checksum comparisons are not performed so we would be depending on the
+ // file size and modification time.
+ if (sameLength && (source.getLen() > 0) && sameBlockSize &&
+ source.getModificationTime() < target.getModificationTime()) {
Review Comment:
Why the addition of the `getLen() > 0`? We want to always copy if its an
empty file?
> Distcp -update between different cloud stores to use modification time while
> checking for file skip.
> ----------------------------------------------------------------------------------------------------
>
> Key: HADOOP-18596
> URL: https://issues.apache.org/jira/browse/HADOOP-18596
> Project: Hadoop Common
> Issue Type: Improvement
> Components: tools/distcp
> Reporter: Mehakmeet Singh
> Assignee: Mehakmeet Singh
> Priority: Major
> Labels: pull-request-available
>
> Distcp -update currently relies on File size, block size, and Checksum
> comparisons to figure out which files should be skipped or copied.
> Since different cloud stores have different checksum algorithms we should
> check for modification time as well to the checks.
> This would ensure that while performing -update if the files are perceived to
> be out of sync we should copy them. The machines between which the file
> transfers occur should be in time sync to avoid any extra copies.
> Improving testing and documentation for modification time checks between
> different object stores to ensure no incorrect skipping of files.
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