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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9381) When same block came for replication for Striped mode, we can move that block to PendingReplications
Date Tue, 01 Dec 2015 02:45:11 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9381?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15032977#comment-15032977
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Jing Zhao commented on HDFS-9381:
---------------------------------

bq. I'm not fully following the above. Jing do you mind elaborating it a little bit?

Sorry for the confusion. As commented by Walter, "if DN_2 fails soon after DN_1, only neededReplications
updated", i.e., the records in neededReplications will have enough time to be updated so that
they can indicate the block groups are missing 2 internal blocks, before the reported issue
happens.



> 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: erasure-coding, namenode
>    Affects Versions: 3.0.0
>            Reporter: Uma Maheswara Rao G
>            Assignee: Uma Maheswara Rao G
>         Attachments: HDFS-9381-02.patch, HDFS-9381-03.patch, HDFS-9381.00.patch, HDFS-9381.01.patch
>
>
> 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 we just return null and if neededReplications contains only fewer blocks(basically
by default if less than numliveNodes*2), then same blocks can be picked again from neededReplications
from next loop 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 but 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 the block is really missed, srcNode will not report it. So this block
will not be removed from pending replications, so that when it is timed out, it will be considered
for replication again and that time it will find actual target to replicate while processing
as part of regular replication flow.



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