Index: tests/Zend/Cache/TwoLevelsBackendTest.php
===================================================================
--- tests/Zend/Cache/TwoLevelsBackendTest.php	(revision 21942)
+++ tests/Zend/Cache/TwoLevelsBackendTest.php	(working copy)
@@ -97,6 +97,33 @@
         ));
     }
 
+    public function testSaveOverwritesIfFastIsFull()
+    {
+        $slowBackend = 'File';
+        $fastBackend = $this->getMock('Zend_Cache_Backend_Apc', array('getFillingPercentage'));
+        $fastBackend->expects($this->at(0))
+            ->method('getFillingPercentage')
+            ->will($this->returnValue(0));
+        $fastBackend->expects($this->at(1))
+            ->method('getFillingPercentage')
+            ->will($this->returnValue(90));
+        $slowBackendOptions = array(
+            'cache_dir' => $this->_cache_dir
+        );
+        $cache = new Zend_Cache_Backend_TwoLevels(array(
+            'fast_backend' => $fastBackend,
+            'slow_backend' => $slowBackend,
+            'slow_backend_options' => $slowBackendOptions,
+            'stats_update_factor' => 1
+        ));
+
+        $id = 'test'.uniqid();
+        $cache->save(10, $id); //fast usage at 0%
+
+        $cache->save(100, $id); //fast usage at 90%
+        $this->assertEquals(100, $cache->load($id));
+    }
+
 }
 
 
Index: library/Zend/Cache/Backend/TwoLevels.php
===================================================================
--- library/Zend/Cache/Backend/TwoLevels.php	(revision 21942)
+++ library/Zend/Cache/Backend/TwoLevels.php	(working copy)
@@ -123,12 +123,26 @@
         parent::__construct($options);
         if ($this->_options['slow_backend'] === null) {
             Zend_Cache::throwException('slow_backend option has to set');
+        } else if (is_string($this->_options['slow_backend'])) {
+            $this->_slowBackend = Zend_Cache::_makeBackend($this->_options['slow_backend'], $this->_options['slow_backend_options'], $this->_options['slow_backend_custom_naming'], $this->_options['slow_backend_autoload']);
+        } else {
+            if ((is_object($this->_options['slow_backend'])) && (in_array('Zend_Cache_Backend_Interface', class_implements($this->_options['slow_backend'])))) {
+                $this->_slowBackend = $this->_options['slow_backend'];
+            } else {
+                Zend_Cache::throwException('slow_backend must be a backend name (string) or an object which implements Zend_Cache_Backend_ExtendedInterface');
+            }
         }
         if ($this->_options['fast_backend'] === null) {
             Zend_Cache::throwException('fast_backend option has to set');
+        } else if (is_string($this->_options['fast_backend'])) {
+            $this->_fastBackend = Zend_Cache::_makeBackend($this->_options['fast_backend'], $this->_options['fast_backend_options'], $this->_options['fast_backend_custom_naming'], $this->_options['fast_backend_autoload']);
+        } else {
+            if ((is_object($this->_options['fast_backend'])) && (in_array('Zend_Cache_Backend_Interface', class_implements($this->_options['fast_backend'])))) {
+                $this->_fastBackend = $this->_options['fast_backend'];
+            } else {
+                Zend_Cache::throwException('fast_backend must be a backend name (string) or an object which implements Zend_Cache_Backend_ExtendedInterface');
+            }
         }
-        $this->_slowBackend = Zend_Cache::_makeBackend($this->_options['slow_backend'], $this->_options['slow_backend_options'], $this->_options['slow_backend_custom_naming'], $this->_options['slow_backend_autoload']);
-        $this->_fastBackend = Zend_Cache::_makeBackend($this->_options['fast_backend'], $this->_options['fast_backend_options'], $this->_options['fast_backend_custom_naming'], $this->_options['fast_backend_autoload']);
         if (!in_array('Zend_Cache_Backend_ExtendedInterface', class_implements($this->_slowBackend))) {
             Zend_Cache::throwException('slow_backend must implement the Zend_Cache_Backend_ExtendedInterface interface');
         }