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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8674) Improve performance of postponed block scans
Date Wed, 16 Dec 2015 16:59:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8674?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15060295#comment-15060295
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Ming Ma commented on HDFS-8674:
-------------------------------

Thanks [~daryn]. Your explanation makes sense. It is good to know the perf difference between
"the # of items < 5M" and "larger number of items". +1 on the patch.

> 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
>
>
> 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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