关于缓存的问题

Question

我用express框架做的后台，当我关闭etag,并设置maxAge时，静态资源有时候出现200 OK (from memory cache) 有时候出现200 OK (from disk cache)，那么这两者有什么区别吗？
还有的时候会出现Status Code:304 Not Modified，这又是怎么回事？不是应该直接从缓存里取数据，不用和服务器交互吗？

代码如下：

app.use(express.static(path.join(__dirname, 'build'), { //build文件夹是打包后的文件夹
     maxAge: 86400000,
     etag: false
}));

network下的一些信息：

1):
General
Request URL:http://localhost:18081/monitor/fmc/assets/js/echarts-theme.js
Request Method:GET
Status Code:304 Not Modified
Remote Address:[::1]:18081

Response Headers
Accept-Ranges:bytes
Cache-Control:public, max-age=86400
Connection:keep-alive
Date:Sat, 24 Dec 2016 09:54:19 GMT
Last-Modified:Tue, 08 Nov 2016 11:31:47 GMT
X-Powered-By:Express

2):
General
Request URL:http://localhost:18081/monitor/fmc/assets/images/bg-out.jpg
Request Method:GET
Status Code:200 OK (from memory cache)
Remote Address:[::1]:18081

Response Headers
Accept-Ranges:bytes
Cache-Control:public, max-age=86400
Connection:keep-alive
Content-Length:446462
Content-Type:image/jpeg
Date:Sat, 24 Dec 2016 09:53:56 GMT
Last-Modified:Sat, 26 Nov 2016 01:16:49 GMT
X-Powered-By:Express