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https://issues.apache.org/jira/browse/HDFS-8674?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15174323#comment-15174323
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Daryn Sharp commented on HDFS-8674:
-----------------------------------
I'll try to rebase again.
> Improve performance of postponed block scans
> --------------------------------------------
>
> Key: HDFS-8674
> URL: https://issues.apache.org/jira/browse/HDFS-8674
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 2.6.0
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
> Priority: Critical
> Attachments: HDFS-8674.patch, HDFS-8674.patch
>
>
> When a standby goes active, it marks all nodes as "stale" which will cause
> block invalidations for over-replicated blocks to be queued until full block
> reports are received from the nodes with the block. The replication monitor
> scans the queue with O(N) runtime. It picks a random offset and iterates
> through the set to randomize blocks scanned.
> The result is devastating when a cluster loses multiple nodes during a
> rolling upgrade. Re-replication occurs, the nodes come back, the excess block
> invalidations are postponed. Rescanning just 2k blocks out of millions of
> postponed blocks may take multiple seconds. During the scan, the write lock
> is held which stalls all other processing.
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