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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-8674) Improve performance of postponed block scans
Date Fri, 26 Jun 2015 16:23:05 GMT
Daryn Sharp created HDFS-8674:

             Summary: 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: HDFS
    Affects Versions: 2.6.0
            Reporter: Daryn Sharp
            Assignee: Daryn Sharp
            Priority: Critical

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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