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From "Zhe Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7621) Erasure Coding: update the Balancer/Mover data migration logic
Date Thu, 28 May 2015 06:27:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7621?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14562357#comment-14562357
] 

Zhe Zhang commented on HDFS-7621:
---------------------------------

Sorry for coming to this review late.

Again, the patch looks reasonable overall. My biggest concern is that the patch changes {{Dispatcher}}
pretty heavily in place. It'd be better if we leave the current logic mostly unchanged and
add striping logic incrementally. This will make the merge review much cleaner and easier.

{{DBlock}} actually just maps a {{Block}} to locations. The added {{DBlockStriped}} does 2
things: 1) maps {{Block}} to locations (inherited from {{DBlock}}); and 2) when given a {{StorageGroup}},
calculates the index of the internal block. How about creating a structure only for purpose
#2? Something like the below:

{code}
private static class GlobalBlockGroupMap {
  private final Map<Block, StripedBlockWithLocations> map = new HashMap<>();
  private int getInternalBlkIdx(Block b, StorageGroup storage) {
    ...
  }
}
{code}

Then the change to existing {{Dispatcher}} code can be simple. Right before sending a move
command to DN, just check the new map to see if we need to translate the block ID. It seems
we can add the check in {{executePendingMove}} for this purpose, but I'm not 100% sure.

> Erasure Coding: update the Balancer/Mover data migration logic
> --------------------------------------------------------------
>
>                 Key: HDFS-7621
>                 URL: https://issues.apache.org/jira/browse/HDFS-7621
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Jing Zhao
>            Assignee: Walter Su
>              Labels: HDFS-7285
>         Attachments: HDFS-7621.001.patch, HDFS-7621.002.patch, HDFS-7621.003.patch, HDFS-7621.004.patch,
HDFS-7621.005.patch, HDFS-7621.006.patch
>
>
> Currently the Balancer/Mover only considers the distribution of replicas of the same
block during data migration: the migration cannot decrease the number of racks. With EC the
Balancer and Mover should also take into account the distribution of blocks belonging to the
same block group.



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