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From "Chao Sun (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-12943) Consistent Reads from Standby Node
Date Wed, 20 Dec 2017 18:32:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16298876#comment-16298876
] 

Chao Sun commented on HDFS-12943:
---------------------------------

bq. Thanks Chao Sun, interesting results! You used only 1 SBN to server reads right?
Yes I used 1 ANN + 1SBN + 1ONN (observer NN).

bq.  In both configurations (with and without stale reads), I assume you were saturating the
system? 
In the stale read case, the RPC queue time on the ANN was less than 5ms, while on ONN it was
between 0 to 30ms. In the non-stale read case, the RPC queue time on ANN was around 130-140ms.
So I guess the ANN was not saturated when stale read is enabled?

bq. Did you use Namesystem unfair locking?
The ANN didn't use unfair locking. The ONN used unfair locking + async audit logging (we have
an internal patch to use log4j 2.x) + async edit logging. Do you think it will make a difference
if unfair locking is used on ANN?

bq. 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.

Yes the test was pretty simple. It is basically:
{code}
loop {
  x = randInt(0, 100)
  if (x < 6) {
    fs.createNewFile(..)
    fs.rename(..)
    fs.delete(..)
  } else if (x < 10) {
    fs.listStatus(..)
  } else if (x < 40) {
    fs.getFileBlockLocations(..)
  } else {
    fs.getFileStatus(..)
  }
}
{code}

The file listing was done on a directory with 2K files. 
Let me know if you have any suggestion on improving this. It's pretty easy to change the code
and re-run the benchmark.

> 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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