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From "Zhe Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14790) Implement a new DFSOutputStream for logging WAL only
Date Tue, 01 Dec 2015 21:58:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15034663#comment-15034663

Zhe Zhang commented on HBASE-14790:

Thanks for the clarification Duo.

bq. Here I'm trying to use only one thread to do all the things 
I see. So you "fanout" for every packet with multiple threads. That does make the logic simpler.
But I guess the logic of bumping gen stamp is still necessary -- imagine a packet is written
to 1st DN but fails to reach 2nd DN. Does the higher level WAL just abandon the file in this
case? Simply closing the file without bumping GS will cause data corruption.

>From performance perspective, {{FanOutOneBlockDFSOutputStream}} makes sure every packet
reaches 3 DNs before proceeding -- it's like calling {{hflush}} for every packet. With slow
DNs this could increase latency quite significantly. But if packet-level flush is indeed desired
in the WAL use case, I think the flow in the patch makes sense.

If coarser grained flushing is desired, then I think we should still separate {{OutputStream}}
and {{DataStreamer}} logics, and we should develop a single-block single-replica fail-stop
{{DataStreamer}} as I commented above.

> 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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