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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6784) Avoid rescan twice in HDFS CacheReplicationMonitor for one FS Op if it calls setNeedsRescan multiple times.
Date Fri, 01 Aug 2014 01:58:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14081811#comment-14081811
] 

Yi Liu commented on HDFS-6784:
------------------------------

Thanks [~cmccabe], let's wait to see whether we need to handle it separately after HDFS-6783.

> Avoid rescan twice in HDFS CacheReplicationMonitor for one FS Op if it calls setNeedsRescan
multiple times.
> -----------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-6784
>                 URL: https://issues.apache.org/jira/browse/HDFS-6784
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: caching
>    Affects Versions: 3.0.0
>            Reporter: Yi Liu
>            Assignee: Yi Liu
>         Attachments: HDFS-6784.001.patch
>
>
> In HDFS CacheReplicationMonitor,  rescan is expensive. Sometimes, {{setNeedsRescan}}
is called multiple times, for example, in FSNamesystem#modifyCacheDirective, there are 3 times.
In monitor thread of CacheReplicationMonitor, if it checks {{needsRescan}} is true, rescan
will happen, but {{needsRescan}} is set to false before real scan. Meanwhile, the 2nd or 3rd
time {{setNeedsResacn}} may set {{needsRescan}} to true. So after the scan finish, in next
loop, a new rescan will be triggered, that's not necessary at all and inefficient for rescan
twice. 



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