Yiqun Lin created HDFS-11464:
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             Summary: Improve the selection in choosing storage for blocks
                 Key: HDFS-11464
                 URL: https://issues.apache.org/jira/browse/HDFS-11464
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: namenode
            Reporter: Yiqun Lin
            Assignee: Yiqun Lin


Currently the logic in choosing storage for blocks is not a good way. It always 
uses the first valid storage of a given StorageType ({{see 
DataNodeDescriptor#chooseStorage4Block}}). This should not be a good selection. 
That means blcoks will always be written to the same volume (first volume) 
until this volume has not available space. This problem is brought up by this 
comment ( 
https://issues.apache.org/jira/browse/HDFS-9807?focusedCommentId=15878382&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15878382
 )

There is one solution from me:

* First, based on existing storages in one node, extract all the valid storages 
into a collection.
* Then, disrupt the order of these vaild storages, get a new collection.
* Finally, get the first storage from the new storages collection.

These steps will be executed in {{DataNodeDescriptor#chooseStorage4Block}} and 
replace current logic. I I think this improvement can be done as a subtask 
under HDFS-11419. Any further comments are welcomed.




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