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https://issues.apache.org/jira/browse/HDFS-8578?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14583285#comment-14583285
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Uma Maheswara Rao G commented on HDFS-8578:
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I am not sure if there was a strong reason behind to do in sequential. But in 
my quick look, it make sense to do in parallel. Vinay had prepared an initial 
patch on this and take a look if you can like that and works for you. 
[~vinayrpet], could you post you patch here as initial pass.  
And also Let's look at ATM, Suresh, Colin comments what they will suggest on 
this further.

> On upgrade, Datanode should process all storage/data dirs in parallel
> ---------------------------------------------------------------------
>
>                 Key: HDFS-8578
>                 URL: https://issues.apache.org/jira/browse/HDFS-8578
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>            Reporter: Raju Bairishetti
>            Priority: Critical
>
> Right now, during upgrades datanode is processing all the storage dirs 
> sequentially. Assume it takes ~20 mins to process a single storage dir then  
> datanode which has ~10 disks will take around 3hours to come up.
> *BlockPoolSliceStorage.java*
> {code}
>    for (int idx = 0; idx < getNumStorageDirs(); idx++) {
>       doTransition(datanode, getStorageDir(idx), nsInfo, startOpt);
>       assert getCTime() == nsInfo.getCTime() 
>           : "Data-node and name-node CTimes must be the same.";
>     }
> {code}
> It would save lots of time during major upgrades if datanode process all 
> storagedirs/disks parallelly.
> Can we make datanode to process all storage dirs parallelly?



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