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https://issues.apache.org/jira/browse/HADOOP-19218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17862588#comment-17862588
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ASF GitHub Bot commented on HADOOP-19218:
-----------------------------------------
virajjasani opened a new pull request, #6916:
URL: https://github.com/apache/hadoop/pull/6916
### Description of PR
Jira: HADOOP-19218
### How was this patch tested?
Manual testing + UT
### For code changes:
- [X] Does the title or this PR starts with the corresponding JIRA issue id
(e.g. 'HADOOP-17799. Your PR title ...')?
- [ ] Object storage: have the integration tests been executed and the
endpoint declared according to the connector-specific documentation?
- [ ] If adding new dependencies to the code, are these dependencies
licensed in a way that is compatible for inclusion under [ASF
2.0](http://www.apache.org/legal/resolved.html#category-a)?
- [ ] If applicable, have you updated the `LICENSE`, `LICENSE-binary`,
`NOTICE-binary` files?
> Avoid DNS lookup while creating IPC Connection object
> -----------------------------------------------------
>
> Key: HADOOP-19218
> URL: https://issues.apache.org/jira/browse/HADOOP-19218
> Project: Hadoop Common
> Issue Type: Improvement
> Reporter: Viraj Jasani
> Assignee: Viraj Jasani
> Priority: Major
>
> Been running HADOOP-18628 in production for quite sometime, everything works
> fine as long as DNS servers in HA are available. Upgrading single NS server
> at a time is also a common case, not problematic. Every DNS lookup takes 1ms
> in general.
> However, recently we encountered a case where 2 out of 4 NS servers went down
> (temporarily but it's a rare case). With small duration DNS cache and 2s of
> NS fallback timeout configured in resolv.conf, now any client performing DNS
> lookup can encounter 4s+ delay. This caused namenode outage as listener
> thread is single threaded and it was not able to keep up with large num of
> unique clients (in direct proportion with num of DNS resolutions every few
> seconds) initiating connection on listener port.
> While having 2 out of 4 DNS servers offline is rare case and NS fallback
> settings could also be improved, it is important to note that we don't need
> to perform DNS resolution for every new connection if the intention is to
> improve the insights into VersionMistmatch errors thrown by the server.
> The proposal is the delay the DNS resolution until the server throws the
> error for incompatible header or version mismatch. This would also help with
> ~1ms extra time spent even for healthy DNS lookup.
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