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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3702) Add an option for NOT writing the blocks locally if there is a datanode on the same box as the client
Date Thu, 10 Mar 2016 19:13:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15189771#comment-15189771

stack commented on HDFS-3702:

Excellent. +1.

Below are nits for if you make a new version of the patch:

Be more forthright in the doc on NO_LOCAL_WRITE. Change "Advice the block not being written
to the local DataNode which is on the same host as the client." to "Advise that a block replica
NOT be written to the local DataNode where 'local' means the same host as the client is being
run on."

I suppose there has to be two declarations of the enum NO_LOCAL_WRITE; i.e. we have to do
the convertion from CreateFlag. NO_LOCAL_WRITE to AddBlockFlag. NO_LOCAL_WRITE

The   protected EnumSet<AddBlockFlag> addBlockFlags() { method is an accessor? Should
it be called getAddBlockFlags?

Whats happening here:

255	    if (!avoidLocalNode || results.size() < numOfReplicas) {
256	      LOG.debug("Fallback to use the default block placement.");

If < numOfReplicas we will start writing local? Add this to release note I'd say.

> Add an option for NOT writing the blocks locally if there is a datanode on the same box
as the client
> -----------------------------------------------------------------------------------------------------
>                 Key: HDFS-3702
>                 URL: https://issues.apache.org/jira/browse/HDFS-3702
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 2.5.1
>            Reporter: Nicolas Liochon
>            Assignee: Lei (Eddy) Xu
>            Priority: Minor
>              Labels: BB2015-05-TBR
>         Attachments: HDFS-3702.000.patch, HDFS-3702.001.patch, HDFS-3702.002.patch, HDFS-3702.003.patch,
HDFS-3702.004.patch, HDFS-3702.005.patch, HDFS-3702.006.patch, HDFS-3702.007.patch
> This is useful for Write-Ahead-Logs: these files are writen for recovery only, and are
not read when there are no failures.
> Taking HBase as an example, these files will be read only if the process that wrote them
(the 'HBase regionserver') dies. This will likely come from a hardware failure, hence the
corresponding datanode will be dead as well. So we're writing 3 replicas, but in reality only
2 of them are really useful.

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