LUCENE-6386: correct javadocs about temp disk space required for forceMerge(1)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1670959 13f79535-47bb-0310-9956-ffa450edef68

lucene/CHANGES.txt

@@ -123,6 +123,10 @@ Bug Fixes
   DocumentsWriterStallControl to prevent hangs during indexing if we
   miss a .notify/All somewhere (Mike McCandless)
 
+* LUCENE-6386: Correct IndexWriter.forceMerge documentation to state
+  that up to 3X (X = current index size) spare disk space may be needed
+  to complete forceMerge(1).  (Robert Muir, Shai Erera, Mike McCandless)
+
 Optimizations
 
 * LUCENE-6183, LUCENE-5647: Avoid recompressing stored fields

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

@@ -1547,14 +1547,15 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable {
    * longer be changed).</p>
    *
    * <p>Note that this requires free space that is proportional
-   * to the size of the index in your Directory (2X if you're
-   * using compound file format). For example, if your index
-   * size is 10 MB then you need an additional 10 MB free for
-   * this to complete (20 MB if you're using compound file
-   * format). This is also affected by the {@link Codec} that
-   * is used to execute the merge, and may result in even a
-   * bigger index. Also, it's best to call {@link #commit()}
-   * afterwards, to allow IndexWriter to free up disk space.</p>
+   * to the size of the index in your Directory: 2X if you are
+   * not using compound file format, and 3X if you are.
+   * For example, if your index size is 10 MB then you need
+   * an additional 20 MB free for this to complete (30 MB if
+   * you're using compound file format). This is also affected
+   * by the {@link Codec} that is used to execute the merge,
+   * and may result in even a bigger index. Also, it's best
+   * to call {@link #commit()} afterwards, to allow IndexWriter
+   * to free up disk space.</p>
    *
    * <p>If some but not all readers re-open while merging
    * is underway, this will cause {@code > 2X} temporary

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

@@ -199,8 +199,8 @@ public class TestIndexWriterForceMerge extends LuceneTestCase {
     assertTrue("forceMerge used too much temporary space: starting usage was "
         + startDiskUsage + " bytes; final usage was " + finalDiskUsage
         + " bytes; max temp usage was " + maxDiskUsage
-        + " but should have been " + (3 * maxStartFinalDiskUsage)
-        + " (= 3X starting usage), BEFORE=" + startListing + "AFTER=" + listFiles(dir), maxDiskUsage <= 3 * maxStartFinalDiskUsage);
+        + " but should have been at most " + (4 * maxStartFinalDiskUsage)
+        + " (= 4X starting usage), BEFORE=" + startListing + "AFTER=" + listFiles(dir), maxDiskUsage <= 4 * maxStartFinalDiskUsage);
     dir.close();
   }