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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