Enable IndexReader to merge tail segments on demand, in RAM, when opening
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Key: LUCENE-983
URL: https://issues.apache.org/jira/browse/LUCENE-983
Project: Lucene - Java
Issue Type: Improvement
Components: Index
Reporter: Michael McCandless
Assignee: Michael McCandless
Priority: Minor
Fix For: 2.3
Spinoff from LUCENE-845.
In LUCENE-845, the IndexWriter must pay a high cost (O(N^2) merge
cost) for keeping the number of segments "always small" in the case
where flushes of very small segments (1 doc as worst case) happen
frequently. This happens in "low latency" applications.
This is because IndexWriter must be ready "at every moment" for an
IndexReader to open the index.
But, if we allow IndexReader to use some RAM (give it a RAM buffer) to
load the long tail of small segments into a RAMDirectory, and then
merge them (in RAM), this allows IndexReader to still have good
performance on the index without IndexWriter paying this high merge
cost. This effectively allows us to optimize the tail segments "on
demand" when a reader needs to use them.
When we combine this with LUCENE-743 (efficient "re-open" of a reader)
then we should be able to efficiently handle low latency applications.
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