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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 Fri, 13 Nov 2015 03:04:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15003469#comment-15003469
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Duo Zhang commented on HBASE-14790:
-----------------------------------

The new implementation will be event-driven which means we could use a callback when sync
so we do not need multiple sync threads any more.

So I think it is a big project if we want to implement this in HDFS since we introduce a new
style of API. Even though we could only implement a new writer first, but I think it is still
very important to design a good general interface first.

So I think we could implement this in HBase first and collect some perf results. Then we could
start talking about move it into HDFS.

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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