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From "Heng Chen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14790) Implement a new DFSOutputStream for logging WAL only
Date Tue, 10 Nov 2015 09:40:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14998312#comment-14998312
] 

Heng Chen commented on HBASE-14790:
-----------------------------------

{quote}
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.
{quote}

Excuse me,  memstore rollback and RS crash cause data inconsistency in HBASE-14004,  what's
the relationship with this issue, how can we avoid it after we rewrite {{DFSOutputStream}}.
 Could you explain in detail? [~Apache9]  
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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