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From "Zhe Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (HDFS-12943) Consistent Reads from Standby Node
Date Wed, 20 Dec 2017 08:55:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16298099#comment-16298099
] 

Zhe Zhang edited comment on HDFS-12943 at 12/20/17 8:54 AM:
------------------------------------------------------------

Thanks [~csun], interesting results! You used only 1 SBN to server reads right? In both configurations
(with and without stale reads), I assume you were saturating the system right? It's interesting
to see that with two NNs serving RPCs (1 ANN + 1 SBN), the throughput actually more than doubled
the throughput with 1 ANN. Did you use Namesystem unfair locking?

If I understand correctly, both your test and the Dynamometer test are more like trace-driven
micro benchmarks, where a container issues a certain type of RPC at given timestamp. Chris
was probably referring to a test job with "real code" like {{if !file_exists(path) then create_file(path)}},
where the blocking relationship between calls are miniced.

[~chris.douglas]: the "natural" increase of write traffic is an interesting question. I don't
think the feature will increase the total amount of write RPCs (a given job will still issue
that many writes overall). Writes within a job could become more bursty but the job itself
will run for shorter. Statistically, the 1000s of jobs on the cluster would probably smooth
out this increased burstiness.


was (Author: zhz):
Thanks [~csun], interesting results!

If I understand correctly, both your test and the Dynamometer test are more like trace-driven
micro benchmarks, where a container issues a certain type of RPC at given timestamp. Chris
was probably referring to a test job with "real code" like {{if !file_exists(path) then create_file(path)}},
where the blocking relationship between calls are miniced.

[~chris.douglas]: the "natural" increase of write traffic is an interesting question. I don't
think the feature will increase the total amount of write RPCs (a given job will still issue
that many writes overall). Writes within a job could become more bursty but the job itself
will run for shorter. Statistically, the 1000s of jobs on the cluster would probably smooth
out this increased burstiness.

> Consistent Reads from Standby Node
> ----------------------------------
>
>                 Key: HDFS-12943
>                 URL: https://issues.apache.org/jira/browse/HDFS-12943
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: hdfs
>            Reporter: Konstantin Shvachko
>         Attachments: ConsistentReadsFromStandbyNode.pdf
>
>
> StandbyNode in HDFS is a replica of the active NameNode. The states of the NameNodes
are coordinated via the journal. It is natural to consider StandbyNode as a read-only replica.
As with any replicated distributed system the problem of stale reads should be resolved. Our
main goal is to provide reads from standby in a consistent way in order to enable a wide range
of existing applications running on top of HDFS.



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