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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 07:16:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15033245#comment-15033245
] 

Zhe Zhang commented on HBASE-14790:
-----------------------------------

On the high level, the key requirements here are very similar to what we need for {{DFSOutputStream}}
in HDFS erasure coding:
# *Fanout write to multiple DNs*
# *Simpler logic without hairy pipeline recovery*

In HDFS-EC we have spent a lot of time trying to fit the above requirements with existing
{{DFSOutputStream}} and {{DataStreamer}}. [~jingzhao] and [~walter.k.su] did a great job on
HDFS-9040 and I'm working on HDFS-9079 to further simplify the logic. HDFS-9079 also tries
to create a *simpler single-block writing logic* in {{StripedDataStreamer}}, which I guess
is also a goal of this JIRA. It's actually also based on an *event-driven* model. But I'm
not sure yet if the events collected by {{BlockMetadataCoordinator}} are the same type of
events needed here.

I'm still reading through Duo's patch and haven't fleshed out full details of how to fulfill
requirements from both sides, but on the high level this looks like a potential synergy. Maybe
we can consider:
# In HDFS project, implement a single-block fail-stop {{DataStreamer}} so at the very least
that part can be shared by both efforts. The new {{DataStreamer}} will stop after writing
a block, and won't attempt to recover from DN failures. [~Apache9] LMK if your patch could
use such a streamer, and focus only on the {{OutputStream}} logic. Actually the {{StripedDataStreamer}}
in the latest HDFS-9079 patch is close to this requirement. But it does have the additional
logic to report and handle failure events. I guess that's inevitable to correctly set block
generation stamps.
# If that goes well, we can explore whether / how to share the fan-out logic with {{DFSStripedOutputStream}}.
The outcome might be a {{DFSParalleOutputStream}}, subclassed by {{DFSParallelStripedOutputStream}}
and {{DFSParallelContiguousOutputStream}} (with better names).
# We can further consider whether it's possible to pass the streamer events collected by {{BlockMetadataCoordinator}}
up to HBase.

> 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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