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https://issues.apache.org/jira/browse/HDFS-9314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15021350#comment-15021350
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Walter Su commented on HDFS-9314:
---------------------------------
Suppose {{first}}, {{second}} are not empty.
Before changes to default policy:
1. choose from {{first}}.
After changes to default policy:
1. choose from {{first}}.
2. If step.1 returns null, fallback to choose from {{second}}.
Suppose first, second are not empty, and suppose in {{first}} there are nodes
on same node-group
Before changes to node-group policy:
1. choose from nodes belonging to same node-group from {{first}}.
After changes to node-group policy (What I thought I should be):
1. choose from nodes belonging to same node-group from {{first}}.
2. If step.1 returns null, fallback to all {{first}}.
3. If step.2 returns null, fallback to choose from {{second}}.
(The step.2~3 is the same as default policy)
_Satisfying the placement policy is the prerequisite for the fallback strategy._
What I meant is, we support fallback in default policy, why not node-group
policy?
Besides, the 004 patch simply returns {{all}}, there is no preference to
{{first}} or {{second}}. We can't say it's a fallback strategy.
> 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,
> HDFS-9314.003.patch, HDFS-9314.004.patch, HDFS-9314.005.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 2), DISK(rack 3), DISK(rack 3)} and the storage
> policy changes to HOT_STORAGE_POLICY_ID, BlockPlacementPolicyDefault's won't
> be able to delete SSD replica.
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