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https://issues.apache.org/jira/browse/HDFS-6784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14080600#comment-14080600
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Yi Liu commented on HDFS-6784:
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Hi Colin
{quote}
This patch adds a place where we hold the CRM lock and try to acquire the FSN
lock, which could lead to deadlock.
{quote}
It's false, the patch is we hold the FSN lock (rescan method will do this,
right?), and then try to get CRM lock to set {{needsRescan}}, so it will *not*
lead to deadlock. (Actually I have considered this)
> 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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