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From "Duo Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14790) Implement a new DFSOutputStream for logging WAL only
Date Wed, 02 Dec 2015 04:04:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15035230#comment-15035230
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Duo Zhang commented on HBASE-14790:
-----------------------------------

basically works. I added a test for it

https://github.com/Apache9/hbase/blob/HBASE-14790/hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/TestFanOutOneBlockDFSOutputStream.java

{quote}
So you "fanout" for every packet with multiple threads. 
{quote}
No, I use only one {{EventLoop}} which means there is only one thread.

{quote}
Simply closing the file without bumping GS will cause data corruption.
{quote}
This does not make sense. What if client crashes before bumping GS?

{quote}
then I think we should still separate OutputStream and DataStreamer logics
{quote}
Maybe you are right but I', not sure what is the correct way since there is no netty based
DFSClient yet? I need to find a way to make it basically work, and then try to abstract the
logic.

Next I will dig into the error handling part. Thanks.



> Implement a new DFSOutputStream for logging WAL only
> ----------------------------------------------------
>
>                 Key: HBASE-14790
>                 URL: https://issues.apache.org/jira/browse/HBASE-14790
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Duo Zhang
>
> The original {{DFSOutputStream}} is very powerful and aims to serve all purposes. But
in fact, we do not need most of the features if we only want to log WAL. For example, we do
not need pipeline recovery since we could just close the old logger and open a new one. And
also, we do not need to write multiple blocks since we could also open a new logger if the
old file is too large.
> And the most important thing is that, it is hard to handle all the corner cases to avoid
data loss or data inconsistency(such as HBASE-14004) when using original DFSOutputStream due
to its complicated logic. And the complicated logic also force us to use some magical tricks
to increase performance. For example, we need to use multiple threads to call {{hflush}} when
logging, and now we use 5 threads. But why 5 not 10 or 100?
> So here, I propose we should implement our own {{DFSOutputStream}} when logging WAL.
For correctness, and also for performance.



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