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From Rémy SAISSY (JIRA) <j...@apache.org>
Subject [jira] [Created] (HDFS-11300) HDFS Nameservices introduces DNS latency inter-dependencies between namenodes
Date Fri, 06 Jan 2017 21:34:58 GMT
Rémy SAISSY created HDFS-11300:
----------------------------------

             Summary: HDFS Nameservices introduces DNS latency inter-dependencies between
namenodes
                 Key: HDFS-11300
                 URL: https://issues.apache.org/jira/browse/HDFS-11300
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs, hdfs-client
    Affects Versions: 2.6.0
            Reporter: Rémy SAISSY


When using HDFS Nameservices, a DNS outage or strong latency on  one of nameservices will
impact any DFSClient or other component which has this nameservice in its hdfs-site.xml. Even
though it doesn't use it.

The issue is due to all nameservices being listed in hdfs-site.xml, and hadoop code trying
to resolve all of them upfront.

To ilustrate the issue, here is the use case we have:
* the 'prod' cluster with its dedicated DNS
* the 'preprod' cluster with its dedicated DNS
* both 'prod' and 'preprod'  hdfs-site.xml files have each other nameservices so that they
can use HDFS as follows:
(from prod) hdfs dfs -ls hdfs://preprod/user/j.doe

A DNS outage on the preprod cluster slow down production jobs being scheduled because of the
upfront resolution of the hdfs-site.xml nameservices entries even though a specific job doesn't
use it.


A solution to fix this issue is to fix the upfront resolution and do it in a lazy way.





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