[jira] [Created] (HDFS-9381) When same block came for replication for Striped mode, we can move that block to PendingReplications

["Uma Maheswara Rao G (JIRA)" <ji...@apache.org>]

Uma Maheswara Rao G created HDFS-9381:
-----------------------------------------

             Summary: When same block came for replication for Striped mode, we can move that block to PendingReplications
                 Key: HDFS-9381
                 URL: https://issues.apache.org/jira/browse/HDFS-9381
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: namenode
    Affects Versions: 3.0.0
            Reporter: Uma Maheswara Rao G
            Assignee: Uma Maheswara Rao G


Currently I noticed that we are just returning null if block already exists in pendingReplications in replication flow for striped blocks.

{code}
if (block.isStriped()) {
      if (pendingNum > 0) {
        // Wait the previous recovery to finish.
        return null;
      }
{code}

 Here if neededReplications contains only fewer blocks(basically by default if less than numliveNodes*2), then same blocks can be picked again from neededReplications if we just return null as we are not removing element from neededReplications. Since this replication process need to take fsnamesystmem lock and do, we may spend some time unnecessarily in every loop. 

So my suggestion/improvement is:
 Instead of just returning null, how about incrementing pendingReplications for this block and remove from neededReplications? and also another point to consider here is, to add into pendingReplications, generally we need target and it is nothing to which node we issued replication command. Later when after replication success and DN reported it, block will be removed from pendingReplications from NN addBlock. 

 So since this is newly picked block from neededReplications, we would not have selected target yet. So which target to be passed to pendingReplications if we add this block.. One Option I am thinking is, how about just passing srcNode itself as target for this special condition? So, anyway if block is really missed, srcNode anyway will not report it. So this block will not be removed from pending replications, so that when it timeout, it will be considered for replication and that time it will find actual target to replicate.





 


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