[jira] [Commented] (SPARK-36810) Handle HDFS read inconsistencies on Spark when observer Namenode is used

["Takanobu Asanuma (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-36810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18060529#comment-18060529 ] 

Takanobu Asanuma commented on SPARK-36810:
------------------------------------------

When we set spark.hadoop.dfs.client.failover.observer.auto-msync-period.* to driver and executors, the value becomes the same for both driver and executors, so we couldn’t set different values per process.

Here’s what worked for us:

hdfs-site.xml
{code:xml}
<property>  
  <name>msync.period</name>  
  <value>-1</value>
</property>  
  
<property>  
  <name>dfs.client.failover.observer.auto-msync-period.<cluster_name></name>  
  <value>${msync.period}</value>  
</property> {code}

spark-defaults.conf
{code}
spark.driver.defaultJavaOptions=-Dmsync.period=0
spark.executor.defaultJavaOptions=-Dmsync.period=1000ms 
{code}

> Handle HDFS read inconsistencies on Spark when observer Namenode is used
> ------------------------------------------------------------------------
>
>                 Key: SPARK-36810
>                 URL: https://issues.apache.org/jira/browse/SPARK-36810
>             Project: Spark
>          Issue Type: Bug
>          Components: Spark Core, SQL
>    Affects Versions: 3.2.0
>            Reporter: Venkata krishnan Sowrirajan
>            Priority: Major
>
> In short, with HDFS HA and with the use of [Observer Namenode|https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/ObserverNameNode.html] the read-after-write consistency is only available when both the write and the read happens from the same client.
> But if the write happens on executor and the read happens on the driver, then the reads would be inconsistent causing application failure issues. This can be fixed by calling `FileSystem.msync` before making any read calls where the client thinks the write could have possibly happened elsewhere.
> This issue is discussed in greater detail in this [discussion|https://mail-archives.apache.org/mod_mbox/spark-dev/202108.mbox/browser] 



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