Ehcache 为啥缓存到磁盘中去

<?xml version="1.0" encoding="ISO-8859-1"?>


name="spring"
maxElementsInMemory="10000"
eternal="false"
timeToIdleSeconds="120"
timeToLiveSeconds="120"
overflowToDisk="true"
maxElementsOnDisk="10000000"
diskPersistent="false"
diskExpiryThreadIntervalSeconds="120"
memoryStoreEvictionPolicy="LRU">

<!--开启扫描-->
<cache:annotation-driven cache-manager="cacheManager"></cache:annotation-driven>

<bean id="temp" class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean">
    <property name="configLocation" value="classpath:ehcache.xml"></property>
</bean>
<bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheCacheManager">
    <property name="cacheManager" ref="temp"></property>
</bean>

我也没有搞懂，磁盘的读写速度不是远远低于内存的读写速度