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From Michael McCandless <>
Subject Re: MergePolicy Thresholds
Date Mon, 02 May 2011 15:34:05 GMT
Actually the new TieredMergePolicy (only on trunk currently but I plan
to backport for 3.2) lets you set the max merged segment size

It's only an "estimate", but if it's set, it tries to pick a merge
reaching around that target size.


On Mon, May 2, 2011 at 9:03 AM, Shai Erera <> 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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