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https://issues.apache.org/jira/browse/HDFS-14069?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16702559#comment-16702559
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Wei-Chiu Chuang commented on HDFS-14069:
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Visibility into decommissioning has been a big problem for us. However I don't
quite understand the purpose of this patch. I am more interested to know the
decomm progress on each DN, rather than the details like the set of blocks, so
I can estimate how much longer it will take to complete the decomm.
> Better debuggability for datanode decommissioning
> -------------------------------------------------
>
> Key: HDFS-14069
> URL: https://issues.apache.org/jira/browse/HDFS-14069
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode, hdfs, namenode
> Reporter: Danny Becker
> Assignee: Danny Becker
> Priority: Major
> Attachments: HDFS-14069.000.patch, HDFS-14069.001.patch
>
>
> Currently, we don't provide any debugging info for decommissioning DN, it is
> difficult to determine which blocks are on their last replica. We have two
> design options:
> # Add block info for blocks with low replication (configurable)
> ** Advantages:
> *** Initial debugging information would be more thorough
> *** Easier initial implementation
> ** Disadvantages:
> *** Add load to normal NN operation by checking every time a DN is
> decommissioned
> *** More difficult to add debugging information later on
> # Create a new api for querying more detailed info about one DN
> ** Advantages:
> *** We wouldnt be adding more load to the NN in normal operation
> *** Much easier to extend in the future with more info
> ** Disadvantages:
> *** Getting the info on demand for this case will be much more expensive
> actually, cause we will have to find all the blocks on that DN, and then go
> through all the blocks again and count how many replicas we have etc.
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