[
https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16427982#comment-16427982
]
Marc Morissette commented on LUCENE-7976:
-----------------------------------------
[~erickerickson] Thanks for tackling this.
Regarding singleton merges: if I read your code correctly and am right about
how Lucene works, I think that, on a large enough collection, your patch could
generate ~50% more reads/writes when re-indexing the whole collection:
* I think new documents are typically flushed once and merged 2-3 times before
ending up in a large segment.
* With a 20% delete threshold, old documents would, on average, be singleton
merged 4 times before being expunged vs only one merge at a 50% delete
threshold. In Latex notation:
{code:java}
20% deleted docs threshold:
\sum_{n=1}^\infnty (1 - 0.2)^n = (1 / (1 - (1 - 0.2))) - 1 = 4
50% deleted docs threshold:
\sum_{n=1}^\infnty (1 - 0.5)^n = (1 / (1 - (1 - 0.5))) - 1 = 1{code}
On the odd chance that my math bears any resemblance to reality, I would
suggest that you disable singleton merges when the short term deletion rate of
a segment is above a certain threshold (say 0.5% per hour). This should prevent
performance degradations during heavy re-indexation while maintaining the
desired behaviour on seldom updated indexes.
> Make TieredMergePolicy respect maxSegmentSizeMB and allow singleton merges of
> very large segments
> -------------------------------------------------------------------------------------------------
>
> Key: LUCENE-7976
> URL: https://issues.apache.org/jira/browse/LUCENE-7976
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
> Attachments: LUCENE-7976.patch, LUCENE-7976.patch
>
>
> 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.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
