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From "Lei (Eddy) Xu (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 22:33:40 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15190091#comment-15190091

Lei (Eddy) Xu commented on HDFS-3702:

Thanks, [~arpitagarwal].

This patch shares many similarities with HDFS-4946, in the following ways:

This patch is orthogonal to storage policy, original block placement policy (i.e., rack-aware
policy). The storage policy, local rack and etc are all honored. This patch is basically adding
one special case for {{excludeNodes}}. I'd say that the only thing it changes is for HDFS-4946,
as an opposite case to HDFS-4946, but only for per-client / DFSOutputStream base.  For the
cases similar in this JIRA, using storage policy alone does not necessarily provide better
data availability (i.e., Hbase still writes to local SSD).

bq. I am also curious about the answer to Devaraj's question. HDFS-2576 was added specifically
for HBase. Can it address your use case? 

To some extend, HDFS-2576 needs each DFSClient have the rest of the cluster in the {{favoriteNodes}}
to achieve the same purpose. It'd also raise question like: would holding a subset of ND in
{{favoriteNodes}} affect the efficiency of data placement? or should DFSClient constantly
refresh this list of nodes? A similar argument can be applied to HDFS-4946 as well.

bq.  The NameNode ignores this CreateFlag.

I am not sure that I understand this question. It is still {{BlockManager}} in {{NameNode}}
making the final decision of block placement (please see my first point). {{CreateFlag}} is
just a user visible flag to provide the _hints_. These (and future more) hints are sent to
NameNode through {{ClientNamenodeProtocol}} RPCs and processed by NameNode.

bq. it will only work for DFSClient users e.g. not for WebHDFS. 

At this time, I am not certain that it will not work for WebHDFS. If that is the case, can
we file a following JIRA to fix it once the basic function is in place?

I hope that the above explanations can answer your questions, [~arpitagarwal]. Looking forward
to hear from you.

> 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, HDFS-3702.008.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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