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From Stack <st...@duboce.net>
Subject Re: Re: Re: Re: What way to improve MTTR other than DLR(distributed log replay)
Date Thu, 27 Oct 2016 22:01:54 GMT
On Fri, Oct 21, 2016 at 3:24 PM, Enis Söztutar <enis.soz@gmail.com> wrote:

> A bit late, but let me give my perspective. This can also be moved to jira
> or dev@ I think.
>
> DLR was a nice and had pretty good gains for MTTR. However, dealing with
> the sequence ids, onlining regions etc and the replay paths proved to be
> too difficult in practice. I think the way forward would be to not bring
> DLR back, but actually fix long standing log split problems.
>
> The main gains in DLR is that we do not create lots and lots of tiny files,
> but instead rely on the regular region flushes, to flush bigger files. This
> also helps with handling requests coming from different log files etc. The
> only gain that I can think of that you get with DLR, but not with log split
> is the online enabling of writes while the recovery is going on. However, I
> think it is not worth having DLR just for this feature.
>
>
And not having to write intermediary files as you note at the start of your
paragraph.



> Now, what are the problems with Log Split you ask. The problems are
>   - we create a lot of tiny files
>   - these tiny files are replayed sequentially when the region is assigned
>   - The region has to replay and flush all data sequentially coming from
> all these tiny files.
>
>
Longest pole in MTTR used to be noticing the RS had gone away in the first
place. Lets not forget to add this to our list.



> In terms of IO, we pay the cost of reading original WAL files, and writing
> this same amount of data into many small files where the NN overhead is
> huge. Then for every region, we do serially sort the data by re-reading the
> tiny WAL files (recovered edits) and sorting them in memory and flushing
> the data. Which means we do 2 times the reads and writes that we should do
> otherwise.
>
> The way to solve our log split bottlenecks is re-reading the big table
> paper and implement the WAL recovery as described there.
>  - Implement an HFile format that can contain data from multiple regions.
> Something like a concatinated HFile format where each region has its own
> section, with its own sequence id, etc.

 - Implement links to these files where a link can refer to this data. This
> is very similar to our ReferenceFile concept.

 - In each log splitter task, instead of generating tiny WAL files that are
> recovered edits, we instead buffer up in memory, and do a sort (this is the
> same sort of inserting into the memstore) per region. A WAL is ~100 MB on
> average, so should not be a big problem to buffer up this.



Need to be able to spill. There will be anomalies.



> At the end of
> the WAL split task, write an hfile containing data from all the regions as
> described above. Also do a multi NN request to create links in regions to
> refer to these files (Not sure whether NN has a batch RPC call or not).
>
>
It does not.

So, doing an accounting, I see little difference from what we have now. In
new scheme:

+ We read all WALs as before.
+ We write about the same (in current scheme, we'd aggregate
across WAL so we didn't write a recovered edits file per WAL) though new
scheme
maybe less since we currently flush after replay of recovered edits so we
nail an
hfile into the file system that has the recovered edits (but in new scheme,
we'll bring
on a compaction because we have references which will cause a rewrite of
the big hfile
into a smaller one...).
+ Metadata ops are about the same (rather than lots of small recovered
edits files instead
we write lots of small reference files)

... only current scheme does distributed, paralellized sort and can spill
if doesn't fit memory.

Am I doing the math right here?

Is there big improvement in MTTR? We are offline while we sort and write
the big hfile and its
references. We might save some because we just open the region after the
above is done where
now we have open and then replay recovered edits (though we could take
writes in current
scheme w/ a bit of work).

Can we do better?

St.Ack



> The reason this will be on-par or better than DLR is that, we are only
> doing 1 read and 1 write, and the sort is parallelized. The region opening
> does not have to block on replaying anything or waiting for flush, because
> the data is already sorted and in HFile format. These hfiles will be used
> the normal way by adding them to the KVHeaps, etc. When compactions run, we
> will be removing the links to these files using the regular mechanisms.
>
> Enis
>
> On Tue, Oct 18, 2016 at 6:58 PM, Ted Yu <yuzhihong@gmail.com> wrote:
>
> > Allan:
> > One factor to consider is that the assignment manager in hbase 2.0 would
> be
> > quite different from those in 0.98 and 1.x branches.
> >
> > Meaning, you may need to come up with two solutions for a single problem.
> >
> > FYI
> >
> > On Tue, Oct 18, 2016 at 6:11 PM, Allan Yang <allanwin@163.com> wrote:
> >
> > > Hi, Ted
> > > These issues I mentioned above(HBASE-13567, HBASE-12743, HBASE-13535,
> > > HBASE-14729) are ALL reproduced in our HBase1.x test environment.
> Fixing
> > > them is exactly what I'm going to do. I haven't found the root cause
> yet,
> > > but  I will update if I find solutions.
> > >  what I afraid is that, there are other issues I don't know yet. So if
> > you
> > > or other guys know other issues related to DLR, please let me know
> > >
> > >
> > > Regards
> > > Allan Yang
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > At 2016-10-19 00:19:06, "Ted Yu" <yuzhihong@gmail.com> wrote:
> > > >Allan:
> > > >I wonder how you deal with open issues such as HBASE-13535.
> > > >From your description, it seems your team fixed more DLR issues.
> > > >
> > > >Cheers
> > > >
> > > >On Mon, Oct 17, 2016 at 11:37 PM, allanwin <allanwin@163.com> wrote:
> > > >
> > > >>
> > > >>
> > > >>
> > > >> Here is the thing. We have backported DLR(HBASE-7006) to our 0.94
> > > >> clusters  in production environment(of course a lot of bugs are
> fixed
> > > and
> > > >> it is working well). It is was proven to be a huge gain. When a
> large
> > > >> cluster crash down, the MTTR improved from several hours to less
> than
> > a
> > > >> hour. Now, we want to move on to HBase1.x, and still we want DLR.
> This
> > > >> time, we don't want to backport the 'backported' DLR to HBase1.x,
> but
> > it
> > > >> seems like that the community have determined to remove DLR...
> > > >>
> > > >>
> > > >> The DLR feature is proven useful in our production environment, so
I
> > > think
> > > >> I will try to fix its issues in branch-1.x
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > >> At 2016-10-18 13:47:17, "Anoop John" <anoop.hbase@gmail.com>
wrote:
> > > >> >Agree with ur observation.. But DLR feature we wanted to get
> > removed..
> > > >> >Because it is known to have issues..  Or else we need major work
to
> > > >> >correct all these issues.
> > > >> >
> > > >> >-Anoop-
> > > >> >
> > > >> >On Tue, Oct 18, 2016 at 7:41 AM, Ted Yu <yuzhihong@gmail.com>
> wrote:
> > > >> >> If you have a cluster, I suggest you turn on DLR and observe
the
> > > effect
> > > >> >> where fewer than half the region servers are up after the
crash.
> > > >> >> You would have first hand experience that way.
> > > >> >>
> > > >> >> On Mon, Oct 17, 2016 at 6:33 PM, allanwin <allanwin@163.com>
> > wrote:
> > > >> >>
> > > >> >>>
> > > >> >>>
> > > >> >>>
> > > >> >>> Yes, region replica is a good way to improve MTTR. Specially
if
> > one
> > > or
> > > >> two
> > > >> >>> servers are down, region replica can improve data availability.
> > But
> > > >> for big
> > > >> >>> disaster like 1/3 or 1/2 region servers shutdown, I think
DLR
> > still
> > > >> useful
> > > >> >>> to bring regions online more quickly and with less IO
usage.
> > > >> >>>
> > > >> >>>
> > > >> >>> Regards
> > > >> >>> Allan Yang
> > > >> >>>
> > > >> >>>
> > > >> >>>
> > > >> >>>
> > > >> >>>
> > > >> >>>
> > > >> >>> At 2016-10-17 21:01:16, "Ted Yu" <yuzhihong@gmail.com>
wrote:
> > > >> >>> >Here was the thread discussing DLR:
> > > >> >>> >
> > > >> >>> >http://search-hadoop.com/m/YGbbOxBK2n4ES12&subj=Re+
> > > >> >>> DISCUSS+retiring+current+DLR+code
> > > >> >>> >
> > > >> >>> >> On Oct 17, 2016, at 4:15 AM, allanwin <allanwin@163.com>
> > wrote:
> > > >> >>> >>
> > > >> >>> >> Hi, All
> > > >> >>> >>  DLR can improve MTTR dramatically, but since
it have many
> bugs
> > > like
> > > >> >>> HBASE-13567, HBASE-12743, HBASE-13535, HBASE-14729(any
more
> > I'don't
> > > >> know?),
> > > >> >>> it was proved unreliable, and has been deprecated almost
in all
> > > >> branches
> > > >> >>> now.
> > > >> >>> >>
> > > >> >>> >>
> > > >> >>> >> My question is, is there any other way other
than DLR to
> > improve
> > > >> MTTR?
> > > >> >>> 'Cause If a big cluster crashes, It takes a long time
to bring
> > > regions
> > > >> >>> online, not to mention it will create huge pressure on
the IOs.
> > > >> >>> >>
> > > >> >>> >>
> > > >> >>> >> To tell the truth, I still want DLR back, if
the community
> > don't
> > > >> have
> > > >> >>> any plan to bring back DLR, I may want to figure out
the
> problems
> > in
> > > >> DLR
> > > >> >>> and make it working and reliable, Any suggests for that?
> > > >> >>> >>
> > > >> >>> >>
> > > >> >>> >> sincerely
> > > >> >>> >> Allan Yang
> > > >> >>>
> > > >>
> > >
> >
>

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