[jira] [Commented] (HDFS-15751) Add documentation for msync() API to filesystem.md

["Konstantin Shvachko (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-15751?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17254662#comment-17254662 ] 

Konstantin Shvachko commented on HDFS-15751:
--------------------------------------------

Added documentation for {{msync()}}.
I put a reference to HDFS documentation describing [Consistent Reads from Observer|https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/ObserverNameNode.html] particulalrly the semantics of {{msync()}} in HDFS. Otherwise it is hard to define it in abstract terms without the context.

I was also thinking about {{AbstractContract}}* type tests for {{msync}} but could not think of anything valuable that can be tested here. Essentially we need call {{mkdir()}} and then verify that after {{msync}} the metadata exists via a read call, which we do for HDFS in {{TestConsistentReadsObserver}}. But it is a probabilistic thing as it can succeed even without synchronization if standby catches up fast enough. I guess testing consistency contracts is similar to atomicity, which we don't test in {{AbstractContract}} tests, since it not clear how.

> Add documentation for msync() API to filesystem.md
> --------------------------------------------------
>
>                 Key: HDFS-15751
>                 URL: https://issues.apache.org/jira/browse/HDFS-15751
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: documentation
>            Reporter: Konstantin Shvachko
>            Assignee: Konstantin Shvachko
>            Priority: Major
>         Attachments: HDFS-15751-01.patch
>
>
> HDFS-15567 introduced new {{FileSystem}} call {{msync()}}. Should add it to the API definitions.



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