Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 966616)
+++ lucene/CHANGES.txt	(working copy)
@@ -241,6 +241,9 @@
   explicitly enable the old behavior with setAutoGeneratePhraseQueries(true) 
   (Robert Muir)
   
+* LUCENE-2537: FSDirectory.copy() implementation was unsafe and could result in 
+  OOM if a large file was copied. (Shai Erera)
+
 New features
 
 * LUCENE-2128: Parallelized fetching document frequencies during weight
Index: lucene/src/java/org/apache/lucene/store/FSDirectory.java
===================================================================
--- lucene/src/java/org/apache/lucene/store/FSDirectory.java	(revision 966616)
+++ lucene/src/java/org/apache/lucene/store/FSDirectory.java	(working copy)
@@ -441,7 +441,20 @@
       try {
         input = new FileInputStream(new File(directory, src)).getChannel();
         output = new FileOutputStream(new File(target.directory, dest)).getChannel();
-        output.transferFrom(input, 0, input.size());
+        /*
+         * If transferFrom is used without a limit when copying a very large
+         * file, then an OOM may be thrown (depends on the state of the RAM in
+         * the machine, as well as the OS used). Performance measurements showed
+         * that chunk sizes larger than 2MB do not result in much faster file
+         * copy, therefore we limit the size to be safe with different file
+         * sizes and systems.
+         */
+        long chunkSize = 1 << 21; // Use 2MB chunk size
+        long numWritten = 0;
+        long numToWrite = input.size();
+        while (numWritten < numToWrite) {
+          numWritten += output.transferFrom(input, numWritten, chunkSize);
+        }
       } catch (IOException ioe) {
         priorException = ioe;
       } finally {