diff -ru jackrabbit-oak-1.7.6/oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/FileIOUtils.java jackrabbit-oak-1.7.6_fixed/oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/FileIOUtils.java
--- jackrabbit-oak-1.7.6/oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/FileIOUtils.java	2017-08-22 11:56:50.000000000 +0200
+++ jackrabbit-oak-1.7.6_fixed/oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/FileIOUtils.java	2017-08-29 17:58:32.728582900 +0200
@@ -95,7 +95,7 @@
      */
     public static void sort(File file, Comparator<String> comparator) throws IOException {
         File sorted = createTempFile("fleioutilssort", null);
-        merge(sortInBatch(file, comparator, true), sorted);
+        merge(sortInBatch(file, comparator, true), sorted, comparator);
         move(sorted, file);
     }
 
@@ -113,6 +113,19 @@
     }
 
     /**
+     * Merges a list of files after sorting with the given comparator.
+     *
+     * @param files files to merge
+     * @param output merge output file
+     * @throws IOException
+     */
+    public static void merge(List<File> files, File output, Comparator<String> comparator) throws IOException {
+        mergeSortedFiles(
+            files,
+            output, comparator, true);
+    }
+
+    /**
 
      * Copies an input stream to a file.
      *
diff -ru jackrabbit-oak-1.7.6/oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/FileIOUtilsTest.java jackrabbit-oak-1.7.6_fixed/oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/FileIOUtilsTest.java
--- jackrabbit-oak-1.7.6/oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/FileIOUtilsTest.java	2017-08-22 11:56:50.000000000 +0200
+++ jackrabbit-oak-1.7.6_fixed/oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/FileIOUtilsTest.java	2017-08-29 17:56:41.792490400 +0200
@@ -40,7 +40,9 @@
 import javax.annotation.Nullable;
 
 import com.google.common.base.Function;
+import com.google.common.collect.Iterators;
 import com.google.common.collect.Sets;
+import com.google.common.primitives.Longs;
 import org.apache.commons.io.FileUtils;
 import org.apache.jackrabbit.oak.commons.FileIOUtils.BurnOnCloseFileIterator;
 import org.junit.Assert;
@@ -65,6 +67,7 @@
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertFalse;
 
 
 /**
@@ -160,6 +163,46 @@
         Collections.sort(list);
         assertArrayEquals(Arrays.toString(list.toArray()), list.toArray(), retrieved.toArray());
     }
+    
+    @Test
+    public void sortLargeFileWithCustomComparatorTest() throws IOException {
+        final int numEntries = 100000;      // must be large enough to trigger split/merge functionality of the sort
+        long[] entries = new long[numEntries];
+        Random r = new Random(0);
+        for (int i = 0; i < numEntries; i++) {
+            entries[i] = r.nextLong();
+        }
+        
+        Iterator<Long> boxedEntries = Longs.asList(entries).iterator();
+        Iterator<String> hexEntries = Iterators.transform(boxedEntries, new Function<Long, String>() {
+                    @Nullable @Override public String apply(@Nullable Long input) {
+                        return Long.toHexString(input);
+                    }
+                });
+        File f = assertWrite(hexEntries, false, numEntries);
+        
+        Comparator<String> prefixComparator = new Comparator<String>() {
+            @Override public int compare(String s1, String s2) {
+                return s1.substring(0, 3).compareTo(s2.substring(0, 3));
+            }
+        };
+        
+        sort(f, prefixComparator);
+        BufferedReader reader =
+            new BufferedReader(new InputStreamReader(new FileInputStream(f), UTF_8));
+        String previous = reader.readLine().substring(0, 3);
+        while (true) {
+            String current = reader.readLine();
+            if (current == null) {
+                break;
+            }
+            current = current.substring(0, 3);
+            assertFalse("Distinct sort didn't filter out duplicates properly.", previous.equals(current));
+            assertTrue("Sort didn't create increasing order", previous.compareTo(current) < 0);
+            previous = current;
+        }
+        closeQuietly(reader);
+    }
 
     @Test
     public void testCopy() throws IOException{