Kihwal Lee updated HDFS-8674:
       Resolution: Fixed
     Hadoop Flags: Reviewed
    Fix Version/s: 3.0.0-alpha2
           Status: Resolved  (was: Patch Available)

Committed to trunk, branch-2 and branch-2.8. Thanks for working on this, Daryn.

> 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
>             Fix For: 2.8.0, 3.0.0-alpha2
>         Attachments: HDFS-8674.2.patch, HDFS-8674.branch-2.patch, 
> HDFS-8674.patch, HDFS-8674.patch, HDFS-8674.trunk.2.patch, 
> HDFS-8674.trunk.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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