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https://issues.apache.org/jira/browse/HDFS-5496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13819247#comment-13819247
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Kihwal Lee commented on HDFS-5496:
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With 125M blocks in BlocksMap, checking all blocks without adding anything to 
the under-replicated blocks queue took about 28 seconds. If there is a large 
number of under-replicated blocks, the initialization time can increase 
significantly. There is also an issue with the under-replicated blocks queue 
that make it worse.

> Make replication queue initialization asynchronous
> --------------------------------------------------
>
>                 Key: HDFS-5496
>                 URL: https://issues.apache.org/jira/browse/HDFS-5496
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Kihwal Lee
>
> 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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