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From "Masatake Iwasaki (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8946) Improve choosing datanode storage for block placement
Date Fri, 28 Aug 2015 08:39:45 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8946?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14718222#comment-14718222
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Masatake Iwasaki commented on HDFS-8946:
----------------------------------------

Thanks for working on this, [~Hitliuyi]. I read the code of BlockPlacementPolicyDefault for
HDFS-8945 recently and comment here while my memory is fresh:-)

bq. Besides, no need to shuffle the storages, since we only need to check according to the
storage type on the datanode once.

Here is my understanding of this. Please correct me if I'm wrong:
   
{{LocatedBlock}} returned by {{ClientProtocol#addBlock}}, {{ClientProtocol#getAdditionalDatanode}}
and {{ClientProtocol#updateBlockForPipeline}} contains storageIDs given by {{BlockPlacementPolicy#chooseTarget}}
but the user of these APIs (which is only DataStreamer) does not uses storageIDs. DataStreamer
just send storage type to DataNode and the DataNode decides which volume to use on its own
by using {{VolumeChoosingPolicy}}.


> Improve choosing datanode storage for block placement
> -----------------------------------------------------
>
>                 Key: HDFS-8946
>                 URL: https://issues.apache.org/jira/browse/HDFS-8946
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Yi Liu
>            Assignee: Yi Liu
>         Attachments: HDFS-8946.001.patch, HDFS-8946.002.patch
>
>
> This JIRA is to:
> Improve chooseing datanode storage for block placement:
> In {{BlockPlacementPolicyDefault}} ({{chooseLocalStorage}}, {{chooseRandom}}), we have
following logic to choose datanode storage to place block.
> For given storage type, we iterate storages of the datanode. But for datanode, it only
cares about the storage type. In the loop, we check according to Storage type and return the
first storage if the storages of the type on the datanode fit in requirement. So we can remove
the iteration of storages, and just need to do once to find a good storage of given type,
it's efficient if the storages of the type on the datanode don't fit in requirement since
we don't need to loop all storages and do the same check.
> Besides, no need to shuffle the storages, since we only need to check according to the
storage type on the datanode once.
> This also improves the logic and make it more clear.
> {code}
>       if (excludedNodes.add(localMachine) // was not in the excluded list
>           && isGoodDatanode(localDatanode, maxNodesPerRack, false,
>               results, avoidStaleNodes)) {
>         for (Iterator<Map.Entry<StorageType, Integer>> iter = storageTypes
>             .entrySet().iterator(); iter.hasNext(); ) {
>           Map.Entry<StorageType, Integer> entry = iter.next();
>           for (DatanodeStorageInfo localStorage : DFSUtil.shuffle(
>               localDatanode.getStorageInfos())) {
>             StorageType type = entry.getKey();
>             if (addIfIsGoodTarget(localStorage, excludedNodes, blocksize,
>                 results, type) >= 0) {
>               int num = entry.getValue();
>               ...
> {code}
> (current logic above)



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