Write-through- Write is done synchronously both to the cache and to the backing store.

　　Write-back (or Write-behind) – Writing is done only to the cache. A modified cache block is written back to the store, just before it is replaced.

　　Write-through(直寫模式)在數據更新時，同時寫入緩存Cache和后端存儲。此模式的優點是操作簡單;缺點是因為數據修改需要同時寫入存儲，數據寫入速度較慢。

　　Write-back(回寫模式)在數據更新時只寫入緩存Cache。只在數據被替換出緩存時，被修改的緩存數據才會被寫到后端存儲。此模式的優點是數據寫入速度快，因為不需要寫存儲;缺點是一旦更新后的數據未被寫入存儲時出現系統掉電的情況，數據將無法找回。

　　Write-misses寫缺失的處理方式

　　對于寫操作，存在寫入緩存缺失數據的情況，這時有兩種處理方式：

　　Write allocate (aka Fetch on write) – Datum at the missed-write location is loaded to cache, followed by a write-hit operation. In this approach, write misses are similar to read-misses.

　　No-write allocate (aka Write-no-allocate, Write around) – Datum at the missed-write location is not loaded to cache, and is written directly to the backing store. In this approach, actually only system reads are being cached.

　　Write allocate方式將寫入位置讀入緩存，然后采用write-hit(緩存命中寫入)操作。寫缺失操作與讀缺失操作類似。

　　No-write allocate方式并不將寫入位置讀入緩存，而是直接將數據寫入存儲。這種方式下，只有讀操作會被緩存。

　　無論是Write-through還是Write-back都可以使用寫缺失的兩種方式之一。只是通常Write-back采用Write allocate方式，而Write-through采用No-write allocate方式;因為多次寫入同一緩存時，Write allocate配合Write-back可以提升性能;而對于Write-through則沒有幫助。

　　處理流程圖

　　Write-through模式處理流程：

　　A Write-Through cache with No-Write Allocation

　　Write-back模式處理流程：

　　A Write-Back cache with Write Allocation