[jira] [Created] (HDFS-5496) Make replication queue initialization asynchronous

["Kihwal Lee (JIRA)" <ji...@apache.org>]

Kihwal Lee created HDFS-5496:
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             Summary: 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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