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Date: Tue, 23 Sep 2014 16:55:54 -0700
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Subject: Re: [VOTE] Merge HDFS-6581 to trunk - Writing to replicas in memory.
From: Arpit Agarwal <aagarwal@hortonworks.com>
To: "hdfs-dev@hadoop.apache.org" <hdfs-dev@hadoop.apache.org>
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I have posted write benchmark results to the Jira.

On Tue, Sep 23, 2014 at 3:41 PM, Arpit Agarwal <aagarwal@hortonworks.com>
wrote:

> Hi Andrew, I said "it is not going to be a substantial fraction of memory
> bandwidth". That is certainly not the same as saying it won't be good or
> there won't be any improvement.
>
> Any time you have transfers over RPC or the network stack you will not get
> close to the memory bandwidth even for intra-host transfers.
>
> I'll add some micro-benchmark results to the Jira shortly.
>
> Thanks,
> Arpit
>
> On Tue, Sep 23, 2014 at 2:33 PM, Andrew Wang <andrew.wang@cloudera.com>
> wrote:
>
>> Hi Arpit,
>>
>> Here is the comment. It was certainly not my intention to misquote anyone.
>>
>>
>> https://issues.apache.org/jira/browse/HDFS-6581?focusedCommentId=14138223&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14138223
>>
>> Quote:
>>
>> It would be nice to see that would could get a substantial fraction of
>> memory bandwidth when writing to a single replica in-memory.
>>
>> The comparison will be interesting but I can tell you without measurement
>> it is not going to be a substantial fraction of memory bandwidth. We are
>> still going through DataTransferProtocol with all the copies and overhead
>> that involves.
>>
>> When the goal is in-memory writes and we are unable to achieve a
>> substantial fraction of memory bandwidth, to me that is "not good
>> performance."
>>
>> I also looked through the subtasks, and AFAICT the only one related to
>> improving this is deferring checksum computation. The benchmarking we did
>> on HDFS-4949 showed that this only really helps when you're down to single
>> copy or zero copies with SCR/ZCR. DTP reads didn't see much of an
>> improvement, so I'd guess the same would be true for DTP writes.
>>
>> I think my above three questions are still open, as well as my question
>> about why we're merging now, as opposed to when the performance of the
>> branch is proven out.
>>
>> Thanks,
>> Andrew
>>
>> On Tue, Sep 23, 2014 at 2:10 PM, Arpit Agarwal <aagarwal@hortonworks.com>
>> wrote:
>>
>> > Andrew, don't misquote me. Can you link the comment where I said
>> > performance wasn't going to be good?
>> >
>> > I will add some add some preliminary write results to the Jira later
>> today.
>> >
>> > > What's the plan to improve write performance?
>> > I described this in response to your and Colin's comments on the Jira.
>> >
>> > For the benefit of folks not following the Jira, the immediate task we'd
>> > like to get done post-merge is moving checksum computation off the write
>> > path. Also see open subtasks of HDFS-6581 for other planned perf
>> > improvements.
>> >
>> > Thanks,
>> > Arpit
>> >
>> >
>> > On Tue, Sep 23, 2014 at 1:07 PM, Andrew Wang <andrew.wang@cloudera.com>
>> > wrote:
>> >
>> > > Hi Arpit,
>> > >
>> > > On HDFS-6581, I asked for write benchmarks on Sep 19th, and you
>> responded
>> > > that the performance wasn't going to be good. However, I thought the
>> > > primary goal of this JIRA was to improve write performance, and write
>> > > performance is listed as the first feature requirement in the design
>> doc.
>> > >
>> > > So, this leads me to a few questions, which I also asked last week on
>> the
>> > > JIRA (I believe still unanswered):
>> > >
>> > > - What's the plan to improve write performance?
>> > > - What kind of performance can we expect after the plan is completed?
>> > > - Can this expected performance be validated with a prototype?
>> > >
>> > > Even with these questions answered, I don't understand the need to
>> merge
>> > > this before the write optimization work is completed. Write perf is
>> > listed
>> > > as a feature requirement, so the branch can reasonably be called not
>> > > feature complete until it's shown to be faster.
>> > >
>> > > Thanks,
>> > > Andrew
>> > >
>> > > On Tue, Sep 23, 2014 at 11:47 AM, Jitendra Pandey <
>> > > jitendra@hortonworks.com>
>> > > wrote:
>> > >
>> > > > +1. I have reviewed most of the code in the branch, and I think its
>> > ready
>> > > > to be merged to trunk.
>> > > >
>> > > >
>> > > > On Mon, Sep 22, 2014 at 5:24 PM, Arpit Agarwal <
>> > aagarwal@hortonworks.com
>> > > >
>> > > > wrote:
>> > > >
>> > > > > HDFS Devs,
>> > > > >
>> > > > > We propose merging the HDFS-6581 development branch to trunk.
>> > > > >
>> > > > > The work adds support to write to HDFS blocks in memory. The
>> target
>> > use
>> > > > > case covers applications writing relatively small, intermediate
>> data
>> > > sets
>> > > > > with low latency. We introduce a new CreateFlag for the existing
>> > > > CreateFile
>> > > > > API. HDFS will subsequently attempt to place replicas of file
>> blocks
>> > in
>> > > > > local memory with disk writes occurring off the hot path. The
>> current
>> > > > > design is a simplification of original ideas from Sanjay Radia on
>> > > > > HDFS-5851.
>> > > > >
>> > > > > Key goals of the feature were minimal API changes to reduce
>> > application
>> > > > > burden and best effort data durability. The feature is optional
>> and
>> > > > > requires appropriate DN configuration from administrators.
>> > > > >
>> > > > > Design doc:
>> > > > >
>> > > > >
>> > > >
>> > >
>> >
>> https://issues.apache.org/jira/secure/attachment/12661926/HDFSWriteableReplicasInMemory.pdf
>> > > > >
>> > > > > Test plan:
>> > > > >
>> > > > >
>> > > >
>> > >
>> >
>> https://issues.apache.org/jira/secure/attachment/12669452/Test-Plan-for-HDFS-6581-Memory-Storage.pdf
>> > > > >
>> > > > > There are 28 resolved sub-tasks under HDFS-6581, 3 open tasks for
>> > > > > tests+Jenkins issues  and 7 open subtasks tracking planned
>> > > improvements.
>> > > > > The latest merge patch is 3300 lines of changed code of which 1300
>> > > lines
>> > > > is
>> > > > > new and updated tests. Merging the branch to trunk will allow HDFS
>> > > > > applications to start evaluating the feature. We will continue
>> work
>> > on
>> > > > > documentation, performance tuning and metrics in parallel with the
>> > vote
>> > > > and
>> > > > > post-merge.
>> > > > >
>> > > > > Contributors to design and code include Xiaoyu Yao, Sanjay Radia,
>> > > > Jitendra
>> > > > > Pandey, Tassapol Athiapinya, Gopal V, Bikas Saha, Vikram Dixit,
>> > Suresh
>> > > > > Srinivas and Chris Nauroth.
>> > > > >
>> > > > > Thanks to Haohui Mai, Colin Patrick McCabe, Andrew Wang, Todd
>> Lipcon,
>> > > > Eric
>> > > > > Baldeschwieler and Vinayakumar B for providing useful feedback on
>> > > > > HDFS-6581, HDFS-5851 and sub-tasks.
>> > > > >
>> > > > > The vote runs for the usual 7 days and will expire at 12am PDT on
>> Sep
>> > > 30.
>> > > > > Here is my +1 for the merge.
>> > > > >
>> > > > > Regards,
>> > > > > Arpit
>> > > > >
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