关于ehcache添加diskSpoolBufferSizeMB参数报错的问题

ehcache.xml配置如下：
<?xml version="1.0" encoding="UTF-8"?>


timeToIdleSeconds="180" timeToLiveSeconds="360" overflowToDisk="true" />

<cache name="playListCache"
    maxElementsInMemory="3"
    eternal="false"
    overflowToDisk="true"
            diskSpoolBufferSizeMB="20"
    diskPersistent="false"
    timeToIdleSeconds="360"
    timeToLiveSeconds="180"
    diskExpiryThreadIntervalSeconds="60" />

我添加diskSpoolBufferSizeMB="20"后，初始化CacheManager的时候就会报错。错误如下：
Error configuring from file:/D:/setup/apache-tomcat-7.0.32/webapps/PlayServer/WEB-INF/classes/ehcache.xml.
Initial cause was Error configuring from input stream. Initial cause was null:15: Element does not allow attribute "diskSpoolBufferSizeMB".
请问是什么原因？