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https://issues.apache.org/jira/browse/HDFS-5496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13845580#comment-13845580
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Kihwal Lee commented on HDFS-5496:
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The following change would have been fine if leaving safe mode and initializing
replication queues were synchronized. It appears {{checkMode()}} can start a
background initialization before leaving the safe mode. Since the queues are
unconditionally cleared right before the following, an on-going initialization
should be stopped and redone.
{code}
- if (!isInSafeMode() ||
- (isInSafeMode() && safeMode.isPopulatingReplQueues())) {
+ // We only need to reprocess the queue in HA mode and not in safemode
+ if (!isInSafeMode() && haEnabled) {
{code}
There have been discussions regarding removing safe mode extension and perhaps
safe mode monitor. That will make the check/logic simpler.
> 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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