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From "Walter Su (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9314) Improve BlockPlacementPolicyDefault's picking of excess replicas
Date Tue, 17 Nov 2015 02:17:11 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15007848#comment-15007848
] 

Walter Su commented on HDFS-9314:
---------------------------------

Shall we consider not to violate the placement policy when we decide to decrease rack counts?
In [HDFS-9313|https://issues.apache.org/jira/browse/HDFS-9313?focusedCommentId=14975592&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14975592]
I agree to choose the SSD, since the remaining 3 DISKs are already on enough racks.

Consider a different case,
||rack_0||rack_1||
|replica_0(DISK)|replica_3(SSD)|
|replica_1(DISK)|
|replica_2(DISK)|
If we remove replica_3, the left are not on enough rack. Is it removing replica_3 the right
decision any more? Shall we make sure not to violate the placement policy, before decide to
decrease rack counts? We can schedule a replication task first, then come back to delete the
SSD.
The old logic will choose from {{exactlyOne}} collection as well, but only if {{moreThanOne}}
is empty. So the old logic doesn't violate the placement policy.

> Improve BlockPlacementPolicyDefault's picking of excess replicas
> ----------------------------------------------------------------
>
>                 Key: HDFS-9314
>                 URL: https://issues.apache.org/jira/browse/HDFS-9314
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ming Ma
>            Assignee: Xiao Chen
>         Attachments: HDFS-9314.001.patch, HDFS-9314.002.patch
>
>
> The test case used in HDFS-9313 identified NullPointerException as well as the limitation
of excess replica picking. If the current replicas are on {SSD(rack r1), DISK(rack 1), DISK(rack
2), DISK(rack 2)} and the storage policy changes to HOT_STORAGE_POLICY_ID, BlockPlacementPolicyDefault's
won't be able to delete SSD replica.



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