Index: src/main/java/org/apache/jackrabbit/core/query/lucene/AbstractIndex.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/query/lucene/AbstractIndex.java	(revision 696655)
+++ src/main/java/org/apache/jackrabbit/core/query/lucene/AbstractIndex.java	(working copy)
@@ -154,9 +154,8 @@
      * in the constructor.
      *
      * @return the directory instance passed in the constructor
-     * @throws IOException
      */
-    Directory getDirectory() throws IOException {
+    Directory getDirectory() {
         return directory;
     }
 
@@ -418,6 +417,17 @@
     }
 
     /**
+     * @return the number of bytes this index occupies in memory.
+     */
+    synchronized long getRamSizeInBytes() {
+        if (indexWriter != null) {
+            return indexWriter.ramSizeInBytes();
+        } else {
+            return 0;
+        }
+    }
+
+    /**
      * Closes the shared reader.
      *
      * @throws IOException if an error occurs while closing the reader.
Index: src/main/java/org/apache/jackrabbit/core/query/lucene/MultiIndex.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/query/lucene/MultiIndex.java	(revision 696656)
+++ src/main/java/org/apache/jackrabbit/core/query/lucene/MultiIndex.java	(working copy)
@@ -412,6 +412,7 @@
      *               <code>next()</code> on this iterator may return
      *               <code>null</code>, to indicate that a node could not be
      *               indexed successfully.
+     * @throws IOException if an error occurs while updating the index.
      */
     synchronized void update(Iterator remove, Iterator add) throws IOException {
         synchronized (updateMonitor) {
@@ -989,7 +990,7 @@
 
     /**
      * Checks if it is needed to commit the volatile index according to {@link
-     * SearchIndex#getMinMergeDocs()}.
+     * SearchIndex#getMaxVolatileIndexSize()}.
      *
      * @return <code>true</code> if the volatile index has been committed,
      *         <code>false</code> otherwise.
@@ -997,7 +998,7 @@
      *                     index.
      */
     private boolean checkVolatileCommit() throws IOException {
-        if (volatileIndex.getNumDocuments() >= handler.getMinMergeDocs()) {
+        if (volatileIndex.getRamSizeInBytes() >= handler.getMaxVolatileIndexSize()) {
             commitVolatileIndex();
             return true;
         }
Index: src/main/java/org/apache/jackrabbit/core/query/lucene/SearchIndex.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/query/lucene/SearchIndex.java	(revision 696655)
+++ src/main/java/org/apache/jackrabbit/core/query/lucene/SearchIndex.java	(working copy)
@@ -217,6 +217,12 @@
     private int minMergeDocs = DEFAULT_MIN_MERGE_DOCS;
 
     /**
+     * The maximum volatile index size in bytes until it is written to disk.
+     * The default value is 1048576 (1MB).
+     */
+    private long maxVolatileIndexSize = 1024 * 1024;
+
+    /**
      * volatileIdleTime config parameter.
      */
     private int volatileIdleTime = 3;
@@ -1866,6 +1872,22 @@
         return similarity.getClass().getName();
     }
 
+    /**
+     * Sets a new maxVolatileIndexSize value.
+     *
+     * @param maxVolatileIndexSize the new value.
+     */
+    public void setMaxVolatileIndexSize(long maxVolatileIndexSize) {
+        this.maxVolatileIndexSize = maxVolatileIndexSize;
+    }
+
+    /**
+     * @return the maxVolatileIndexSize in bytes.
+     */
+    public long getMaxVolatileIndexSize() {
+        return maxVolatileIndexSize;
+    }
+
     //----------------------------< internal >----------------------------------
 
     /**
Index: src/main/java/org/apache/jackrabbit/core/query/lucene/VolatileIndex.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/query/lucene/VolatileIndex.java	(revision 696655)
+++ src/main/java/org/apache/jackrabbit/core/query/lucene/VolatileIndex.java	(working copy)
@@ -115,7 +115,7 @@
      *
      * @return the number of valid documents in this index.
      */
-    int getNumDocuments() throws IOException {
+    int getNumDocuments() {
         return numDocs;
     }
 
@@ -142,6 +142,13 @@
     }
 
     /**
+     * {@inheritDoc}
+     */
+    long getRamSizeInBytes() {
+        return super.getRamSizeInBytes() + ((RAMDirectory) getDirectory()).sizeInBytes();
+    }
+
+    /**
      * Sets a new buffer size for pending documents to add to the index.
      * Higher values consume more memory, but help to avoid multiple index
      * cycles when a node is changed / saved multiple times.
@@ -154,6 +161,8 @@
 
     /**
      * Commits pending documents to the index.
+     *
+     * @throws IOException if committing pending documents fails.
      */
     private void commitPending() throws IOException {
         if (pending.isEmpty()) {