1. write back和write through

这里引用smartcache的关于它两的定义：

Write-through Cache Policy: The goal of write-through caching is to accelerate the read operations. All write operations go to
Primary Source (HDDs); write operations may also go to the Cache (SSDs). Thus the write operations may be slower compared to a
configuration without Write-through Cache

Write-back Cache Policy: The goal of write-back caching is to accelerate both read and write operations. Writes may be cached on the Cache (SSDs) and written to the Primary Storage (HDDs) at a later point of time.

翻译：

write-through(透写): 加速读操作，所有的写操作直接写往主存，同时也写在缓存cache中，因此写操作相对较慢。

write-back(回写):同时加速读写操作，写操作先缓存在cache中，稍后才会写到主存中去。

1.1. write-back的优缺点

优点：回写操作先将数据写到cache缓冲之中便立刻返回，不等待磁盘等存储设备IO写指令执行完毕。因为免去了等待操作较慢的IO写操作的执行，这种方式具有较高的效率。

缺点：Cache中会保存上一次写之后的数据（通常叫做脏数据），而且如果写的过程中发生了掉电，则不能确保数据切实被写入到磁盘中。