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https://issues.apache.org/jira/browse/HDFS-5496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13846751#comment-13846751
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Kihwal Lee commented on HDFS-5496:
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> If I understand correctly, Need to restart initializing queues right?
Yes. Since the initialization is now asynchronous, a new kind of problem can
occur. A standby node in safe mode can transition to active, checkMode()
causes the initialization to start (i.e. entering safe mode extension). At this
point, if it transitions back to standby and again to active while in the safe
mode extension period, {{if (!isInSafeMode() && haEnabled)}} will fail to
detect the need to restart the initialization.
> Make replication queue initialization asynchronous
> --------------------------------------------------
>
> Key: HDFS-5496
> URL: https://issues.apache.org/jira/browse/HDFS-5496
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Reporter: Kihwal Lee
> Assignee: Vinay
> Attachments: HDFS-5496.patch, HDFS-5496.patch
>
>
> Today, initialization of replication queues blocks safe mode exit and certain
> HA state transitions. For a big name space, this can take hundreds of seconds
> with the FSNamesystem write lock held. During this time, important requests
> (e.g. initial block reports, heartbeat, etc) are blocked.
> The effect of delaying the initialization would be not starting replication
> right away, but I think the benefit outweighs. If we make it asynchronous,
> the work per iteration should be limited, so that the lock duration is
> capped.
> If full/incremental block reports and any other requests that modifies block
> state properly performs replication checks while the blocks are scanned and
> the queues populated in background, every block will be processed. (Some may
> be done twice) The replication monitor should run even before all blocks are
> processed.
> This will allow namenode to exit safe mode and start serving immediately even
> with a big name space. It will also reduce the HA failover latency.
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