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From Shai Erera <ser...@gmail.com>
Subject Re: MergePolicy Thresholds
Date Mon, 02 May 2011 18:16:40 GMT
Thanks Mike. I'll take a look at TieredMP. Does it depend on trunk in any
way, or do you think it can easily be ported to 3x?

Shai

On Mon, May 2, 2011 at 6:34 PM, Michael McCandless <
lucene@mikemccandless.com> wrote:

> Actually the new TieredMergePolicy (only on trunk currently but I plan
> to backport for 3.2) lets you set the max merged segment size
> (maxMergedSegmentMB).
>
> It's only an "estimate", but if it's set, it tries to pick a merge
> reaching around that target size.
>
> Mike
>
> http://blog.mikemccandless.com
>
> On Mon, May 2, 2011 at 9:03 AM, Shai Erera <serera@gmail.com> wrote:
> > Hi
> >
> > Today, LogMP allows you to set different thresholds for segments sizes,
> > thereby allowing you to control the largest segment that will be
> > considered for merge + the largest segment your index will hold (=~
> > threshold * mergeFactor).
> >
> > So, if you want to end up w/ say 20GB segments, you can set
> > maxMergeMB(ForOptimize) to 2GB and mergeFactor=10.
> >
> > However, this often does not achieve your desired goal -- if the index
> > contains 5 and 7 GB segments, they will never be merged b/c they are
> > bigger than the threshold. I am willing to spend the CPU and IO resources
> > to end up w/ 20 GB segments, whether I'm merging 10 segments together or
> > only 2. After I reach a 20GB segment, it can rest peacefully, at least
> > until I increase the threshold.
> >
> > So I wonder, first, if this threshold (i.e., largest segment size you
> > would like to end up with) is more natural to set than thee current
> > thresholds,
> > from the application level? I.e., wouldn't it be a simpler threshold to
> set
> > instead of doing weird calculus that depend on maxMergeMB(ForOptimize)
> > and mergeFactor?
> >
> > Second, should this be an addition to LogMP, or a different
> > type of MP. One that adheres to only those two factors (perhaps the
> > segSize threshold should be allowed to set differently for optimize and
> > regular merges). It can pick segments for merge such that it maximizes
> > the result segment size (i.e., don't necessarily merge in sequential
> > order), but not more than mergeFactor.
> >
> > I guess, if we think that maxResultSegmentSizeMB is more intuitive than
> > the current thresholds, application-wise, then this change should go
> > into LogMP. Otherwise, it feels like a different MP is needed, because
> > LogMP is already complicated and another threshold would confuse things.
> >
> > What do you think of this? Am I trying to optimize too much? :)
> >
> > Shai
> >
> >
>
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