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https://issues.apache.org/jira/browse/HDFS-13688?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16545898#comment-16545898
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Chen Liang commented on HDFS-13688:
-----------------------------------

I was rebasing the patch, and I found that one of my tests failed. After 
looking into it, it seems that it was caused by that in the final committed 
version of HDFS-12976, {{isObserverState}} has changed. It gets called on 
initialization of {{ObserverReadProxyProvider}}. In current new 
{{isObserverState}} call, it makes an RPC call to every NameNode. As a side 
effect, this RPC call will also set up client side state id. In which case, 
even for a fresh client, it will have state id set after 
{{ObserverReadProxyProvider}} gets created, so there is no longer the need to 
explicitly making a call to set state id for fresh clients. I will post another 
patch to reflect this change.

> Introduce msync API call
> ------------------------
>
>                 Key: HDFS-13688
>                 URL: https://issues.apache.org/jira/browse/HDFS-13688
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Chen Liang
>            Assignee: Chen Liang
>            Priority: Major
>         Attachments: HDFS-13688-HDFS-12943.WIP.002.patch, 
> HDFS-13688-HDFS-12943.WIP.patch
>
>
> As mentioned in the design doc in HDFS-12943, to ensure consistent read, we 
> need to introduce an RPC call {{msync}}. Specifically, client can issue a 
> msync call to Observer node along with a transactionID. The msync will only 
> return when the Observer's transactionID has caught up to the given ID. This 
> JIRA is to add this API.



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