[jira] [Created] (HDFS-8884) Improve BlockPlacementPolicyDefault#chooseTarget to check candidate datanode first

["Yi Liu (JIRA)" <ji...@apache.org>]

Yi Liu created HDFS-8884:
----------------------------

             Summary: Improve BlockPlacementPolicyDefault#chooseTarget to check candidate datanode first 
                 Key: HDFS-8884
                 URL: https://issues.apache.org/jira/browse/HDFS-8884
             Project: Hadoop HDFS
          Issue Type: Improvement
            Reporter: Yi Liu
            Assignee: Yi Liu


In current BlockPlacementPolicyDefault, when choosing datanode storage to place block, we have following logic:

{code}
        final DatanodeStorageInfo[] storages = DFSUtil.shuffle(
            chosenNode.getStorageInfos());
        int i = 0;
        boolean search = true;
        for (Iterator<Map.Entry<StorageType, Integer>> iter = storageTypes
            .entrySet().iterator(); search && iter.hasNext(); ) {
          Map.Entry<StorageType, Integer> entry = iter.next();
          for (i = 0; i < storages.length; i++) {
            StorageType type = entry.getKey();
            final int newExcludedNodes = addIfIsGoodTarget(storages[i],
{code}
We will iterate (actually two {{for}}) all storages of the candidate datanode even the datanode itself is not good (e.g. decommissioned, stale, too busy..), since currently we do all the check in {{addIfIsGoodTarget}}.

We can fail-fast: check the datanode related conditions first, if the datanode is not good, then no need to shuffle and iterate the storages. Then it's more efficient.



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