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https://issues.apache.org/jira/browse/HADOOP-18596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17677623#comment-17677623
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Ayush Saxena commented on HADOOP-18596:
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Just bumped into it while checking my mail and have a question here:
{quote}The machines between which the file transfers occur should be in time
sync to avoid any extra copies.
{quote}
How would you ensure that they are in sync, two clocks *perfectly* in sync is
kind of looks tough.
Extra copy ain't an issue, it will be hitting the performance only, not copying
and having stale data is an issue.
Do you plan to introduce an additional option for this or make it a default
> 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
>
> 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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