Index: src/java/org/apache/lucene/index/CompoundFileWriter.java
===================================================================
--- src/java/org/apache/lucene/index/CompoundFileWriter.java	(revision 613824)
+++ src/java/org/apache/lucene/index/CompoundFileWriter.java	(working copy)
@@ -156,13 +156,23 @@
             // Remember the positions of directory entries so that we can
             // adjust the offsets later
             Iterator it = entries.iterator();
+            long totalSize = 0;
             while(it.hasNext()) {
                 FileEntry fe = (FileEntry) it.next();
                 fe.directoryOffset = os.getFilePointer();
                 os.writeLong(0);    // for now
                 os.writeString(fe.file);
+                totalSize += directory.fileLength(fe.file);
             }
 
+            // Pre-allocate size of file as optimization --
+            // this can potentially help IO performance as
+            // we write the file and also later during
+            // searching.  It also uncovers a disk-full
+            // situation earlier and hopefully without
+            // actually filling disk to 100%:
+            os.setLength(totalSize+os.getFilePointer());
+
             // Open the files and copy their data into the stream.
             // Remember the locations of each file's data section.
             byte buffer[] = new byte[16384];
Index: src/java/org/apache/lucene/store/IndexOutput.java
===================================================================
--- src/java/org/apache/lucene/store/IndexOutput.java	(revision 613824)
+++ src/java/org/apache/lucene/store/IndexOutput.java	(working copy)
@@ -189,5 +189,15 @@
   /** The number of bytes in the file. */
   public abstract long length() throws IOException;
 
-
+  /** Set the file length. By default, this method does
+   * nothing (it's optional for a Directory to implement
+   * it).  But, certain Directory implementations (for
+   * example @see FSDirectory) can use this to inform the
+   * underlying IO system to pre-allocate the file to the
+   * specified size.  If the length is longer than the
+   * current file length, the bytes added to the file are
+   * undefined.  Otherwise the file is truncated.
+   * @param length file length
+   */
+  public void setLength(long length) throws IOException {};
 }
Index: src/java/org/apache/lucene/store/FSDirectory.java
===================================================================
--- src/java/org/apache/lucene/store/FSDirectory.java	(revision 613824)
+++ src/java/org/apache/lucene/store/FSDirectory.java	(working copy)
@@ -615,6 +615,8 @@
     public long length() throws IOException {
       return file.length();
     }
-  
+    public void setLength(long length) throws IOException {
+      file.setLength(length);
+    }
   }
 }