Erick Erickson commented on LUCENE-8263:

+1 to the patch.

There were some weird edge cases in TestTieredMergePolicy that only came to 
light when I beasted it, so I'll run a few thousand iterations over the weekend 
and report back if any pop out.

Your simulations numbers square pretty well with mine when I was doing this one 
at the same time as 7976. 

I originally advocated  _not_ putting a floor on the percentage and providing 
users with one more way to shoot themselves in the foot. I've changed my mind 
on that, I think 20% is fine. Now that they can forceMerge or expungeDeletes 
without creating massive segments, I don't think there's any good (or even bad) 
reason to allow < 20%.

Thanks again for working on this and your help with 7976. Much appreciated.

> Add indexPctDeletedTarget as a parameter to TieredMergePolicy to control more 
> aggressive merging
> ------------------------------------------------------------------------------------------------
>                 Key: LUCENE-8263
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8263
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Erick Erickson
>            Assignee: Erick Erickson
>            Priority: Major
>         Attachments: LUCENE-8263.patch
> Spinoff of LUCENE-7976 to keep the two issues separate.
> The current TMP allows up to 50% deleted docs, which can be wasteful on large 
> indexes. This parameter will do more aggressive merging of segments with 
> deleted documents when the _total_ percentage of deleted docs in the entire 
> index exceeds it.
> Setting this to 50% should approximate current behavior. Setting it to 20% 
> caused the first cut at this to increase I/O roughly 10%. Setting it to 10% 
> caused about a 50% increase in I/O.
> I was conflating the two issues, so I'll change 7976 and comment out the bits 
> that reference this new parameter. After it's checked in we can bring this 
> back. That should be less work than reconstructing this later.
> Among the questions to be answered:
> 1> what should the default be? I propose 20% as it results in significantly 
> less space wasted and helps control heap usage for a modest increase in I/O.
> 2> what should the floor be? I propose 10% with _strong_ documentation 
> warnings about not setting it below 20%.
> 3> should there be two parameters? I think this was discussed somewhat in 
> 7976. The first cut at  this used this number for two purposes:
> 3a> the total percentage of deleted docs index-wide to trip this trigger
> 3b> the percentage of an _individual_ segment that had to be deleted if the 
> segment was over maxSegmentSize/2 bytes in order to be eligible for merging. 
> Empirically, using the same percentage for both caused the merging to hover 
> around the value specified for this parameter.
> My proposal for <3> would be to have the parameter do double-duty. Assuming 
> my preliminary results hold, you specify this parameter at, say, 20% and once 
> the index hits that % deleted docs it hovers right around there, even if 
> you've forceMerged earlier down to 1 segment. This seems in line with what 
> I'd expect and adding another parameter seems excessively complicated to no 
> good purpose. We could always add something like that later if we wanted.

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