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From "Arun Suresh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-7101) Revisit behavior of JHS scan file behavior
Date Tue, 12 Jun 2018 21:04:00 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-7101?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16510203#comment-16510203

Arun Suresh commented on MAPREDUCE-7101:

Thanks [~tmarquardt].
The patch looks good to me. +1  The comment describing the new field in JHAdminConfig is wrong
- minor thing I can fix before committing.

Given that this patch retains the default behavior, and specific cloud deployments can choose
to always scan.
Maybe a pluggable FS specific scan is probably a better long term solution, but I agree with
[~leftnoteasy] and [~rohithsharma] that we should go ahead with the approach in this patch
to unblock.

> Revisit behavior of JHS scan file behavior
> ------------------------------------------
>                 Key: MAPREDUCE-7101
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-7101
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>            Reporter: Wangda Tan
>            Assignee: Thomas Marquardt
>            Priority: Critical
>         Attachments: MAPREDUCE-7101.001.patch
> Currently, the JHS scan directory if the modification of *directory* changed: 
> {code} 
>     public synchronized void scanIfNeeded(FileStatus fs) {
>       long newModTime = fs.getModificationTime();
>       if (modTime != newModTime) {
>         <... omitted some logics ...>
>         // reset scanTime before scanning happens
>         scanTime = System.currentTimeMillis();
>         Path p = fs.getPath();
>         try {
>           scanIntermediateDirectory(p);
> {code}
> This logic relies on an assumption that, the directory's modification time will be updated
if a file got placed under the directory.
> However, the semantic of directory's modification time is not consistent in different
FS implementations. For example, MAPREDUCE-6680 fixed some issues of truncated modification
time. And HADOOP-12837 mentioned on S3, the directory's modification time is always 0.
> I think we need to revisit behavior of this logic to make it to more robustly work on
different file systems.

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