Bump floor segment size to 16MB. (#14189)

This bumps the floor segment size from 2MB (`TieredMergePolicy`) / 1.6MB
(`LogByteSizeMergePolicy`) to 16MB.

My motivation is that such small segment sizes don't make index
structures actually helpful vs. linear scans, so we should avoid them.
Furthermore, there has been progress on merging rules for segments below
the floor size, in particular merge policies no longer perform quadratic
merging (#900) so this change will not make indexing/merging absurdly
slow if an application flushes tiny segments.

Author: jpountz

lucene/CHANGES.txt

@@ -178,6 +178,11 @@ Bug Fixes
 
 * GITHUB#14215: Fix degenerate case in HNSW where all vectors have the same score. (Ben Trent)
 
+Changes in Runtime Behavior
+---------------------
+* GITHUB#14189: Bump floor segment size to 16MB in TieredMergePolicy and
+  LogByteSizeMergePolicy. (Adrien Grand)
+
 Other
 ---------------------
 

lucene/core/src/java/org/apache/lucene/index/LogByteSizeMergePolicy.java

@@ -25,7 +25,7 @@
 public class LogByteSizeMergePolicy extends LogMergePolicy {
 
   /** Default minimum segment size. @see setMinMergeMB */
-  public static final double DEFAULT_MIN_MERGE_MB = 1.6;
+  public static final double DEFAULT_MIN_MERGE_MB = 16;
 
   /**
    * Default maximum segment size. A segment of this size or larger will never be merged. @see

lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java

@@ -84,7 +84,7 @@ public class TieredMergePolicy extends MergePolicy {
 
   private long maxMergedSegmentBytes = 5 * 1024 * 1024 * 1024L;
 
-  private long floorSegmentBytes = 2 * 1024 * 1024L;
+  private long floorSegmentBytes = 16 * 1024 * 1024L;
   private double segsPerTier = 10.0;
   private double forceMergeDeletesPctAllowed = 10.0;
   private double deletesPctAllowed = 20.0;
@@ -166,7 +166,7 @@ public double getDeletesPctAllowed() {
    * </ul>
    *
    * In both cases, this helps prevent frequent flushing of tiny segments to create a long tail of
-   * small segments in the index. Default is 2MB.
+   * small segments in the index. Default is 16MB.
    */
   public TieredMergePolicy setFloorSegmentMB(double v) {
     if (v <= 0.0) {