关于index.translog.durability的疑问

index.translog.durability有两个参数，request和async,我对这两个参数都不是很理解。
官方文档7.7版本https://www.elastic.co/guide/e ... ml%23
request的解释是
(default) fsync and commit after every request. In the event of hardware failure, all acknowledged writes will already have been committed to disk.
这里是指什么意思啊？是指每个请求都执行一次fsync，可是不是说fsnyc代价大，要放到translog中吗？
然后第二个参数async解释是
fsync and commit in the background every sync_interval. In the event of a failure, all acknowledged writes since the last automatic commit will be discarded.
如果选择了这个参数后又会导致什么变化呢？
看了很久都没搞懂，来论坛问问大家，谢谢了。