GITHUB-11761: Move minimum TieredMergePolicy delete percentage and change default value (#11831)

Move minimum TieredMergePolicy delete percentage from 20% to 5%

and change deletePctAllowed default to 20%

Co-authored-by: Marc D'Mello <dmellomd@amazon.com>

lucene/CHANGES.txt

@@ -98,6 +98,9 @@ API Changes
 
 * GITHUB#11804: FacetsCollector#collect is no longer final, allowing extension. (Greg Miller)
 
+* GITHUB#11761: TieredMergePolicy now allowed a maximum allowable deletes percentage of down to 5%, and the default
+  maximum allowable deletes percentage is changed from 33% to 20%. (Marc D'Mello)
+
 Improvements
 ---------------------
 * GITHUB#11778: Detailed part-of-speech information for particle(조사) and ending(어미) on Nori

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

@@ -92,7 +92,7 @@ public class TieredMergePolicy extends MergePolicy {
   private long floorSegmentBytes = 2 * 1024 * 1024L;
   private double segsPerTier = 10.0;
   private double forceMergeDeletesPctAllowed = 10.0;
-  private double deletesPctAllowed = 33.0;
+  private double deletesPctAllowed = 20.0;
 
   /** Sole constructor, setting all settings to their defaults. */
   public TieredMergePolicy() {
@@ -151,12 +151,17 @@ public class TieredMergePolicy extends MergePolicy {
   /**
    * Controls the maximum percentage of deleted documents that is tolerated in the index. Lower
    * values make the index more space efficient at the expense of increased CPU and I/O activity.
-   * Values must be between 20 and 50. Default value is 33.
+   * Values must be between 5 and 50. Default value is 20.
+   *
+   * <p>When the maximum delete percentage is lowered, the indexing thread will call for merges more
+   * often, meaning that write amplification factor will be increased. Write amplification factor
+   * measures the number of times each document in the index is written. A higher write
+   * amplification factor will lead to higher CPU and I/O activity as indicated above.
    */
   public TieredMergePolicy setDeletesPctAllowed(double v) {
-    if (v < 20 || v > 50) {
+    if (v < 5 || v > 50) {
       throw new IllegalArgumentException(
-          "indexPctDeletedTarget must be >= 20.0 and <= 50 (got " + v + ")");
+          "indexPctDeletedTarget must be >= 5.0 and <= 50 (got " + v + ")");
     }
     deletesPctAllowed = v;
     return this;

lucene/core/src/test/org/apache/lucene/index/TestIndexWriterDelete.java

@@ -1345,6 +1345,7 @@ public class TestIndexWriterDelete extends LuceneTestCase {
     w.addDocument(doc);
     w.close();
     iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+    ((TieredMergePolicy) iwc.getMergePolicy()).setDeletesPctAllowed(33.0);
     iwc.setOpenMode(IndexWriterConfig.OpenMode.APPEND);
     w = new IndexWriter(d, iwc);
     IndexReader r = DirectoryReader.open(w, false, false);

lucene/core/src/test/org/apache/lucene/index/TestTieredMergePolicy.java

@@ -310,6 +310,7 @@ public class TestTieredMergePolicy extends BaseMergePolicyTestCase {
             ((1024.0 * 1024.0)); // fudge it up, we're trying to catch egregious errors and segbytes
     // don't really reflect the number for original merges.
     tmp.setMaxMergedSegmentMB(mbSize);
+    tmp.setDeletesPctAllowed(33.0);
     conf.setMaxBufferedDocs(100);
     conf.setMergePolicy(tmp);