Erick Erickson created LUCENE-7976:
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Summary: Add a parameter to TieredMergePolicy to merge segments
that have more than X percent deleted documents
Key: LUCENE-7976
URL: https://issues.apache.org/jira/browse/LUCENE-7976
Project: Lucene - Core
Issue Type: Improvement
Reporter: Erick Erickson
We're seeing situations "in the wild" where there are very large indexes (on
disk) handled quite easily in a single Lucene index. This is particularly true
as features like docValues move data into MMapDirectory space. The current TMP
algorithm allows on the order of 50% deleted documents as per a dev list
conversation with Mike McCandless (and his blog here:
https://www.elastic.co/blog/lucenes-handling-of-deleted-documents).
Especially in the current era of very large indexes in aggregate, (think many
TB) solutions like "you need to distribute your collection over more shards"
become very costly. Additionally, the tempting "optimize" button exacerbates
the issue since once you form, say, a 100G segment (by optimizing/forceMerging)
it is not eligible for merging until 97.5G of the docs in it are deleted
(current default 5G max segment size).
The proposal here would be to add a new parameter to TMP, something like
<maxAllowedPctDeletedInBigSegments> (no, that's not serious name, suggestions
welcome) which would default to 100 (or the same behavior we have now).
So if I set this parameter to, say, 20%, and the max segment size stays at 5G,
the following would happen when segments were selected for merging:
> any segment with > 20% deleted documents would be merged or rewritten NO
> MATTER HOW LARGE. There are two cases,
>> the segment has < 5G "live" docs. In that case it would be merged with
>> smaller segments to bring the resulting segment up to 5G. If no smaller
>> segments exist, it would just be rewritten
>> The segment has > 5G "live" docs (the result of a forceMerge or optimize).
>> It would be rewritten into a single segment removing all deleted docs no
>> matter how big it is to start. The 100G example above would be rewritten to
>> an 80G segment for instance.
Of course this would lead to potentially much more I/O which is why the default
would be the same behavior we see now. As it stands now, though, there's no way
to recover from an optimize/forceMerge except to re-index from scratch. We
routinely see 200G-300G Lucene indexes at this point "in the wild" with 10s of
shards replicated 3 or more times. And that doesn't even include having these
over HDFS.
Alternatives welcome! Something like the above seems minimally invasive. A new
merge policy is certainly an alternative.
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