Cache寫機制
參考http://witmax.cn/cache-writing-policies.html上的說明，Cache寫機制分為write through和write back兩種。
- 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(shù)據(jù)更新時，同時寫入緩存Cache和后端存儲。此模式的優(yōu)點是操作簡單；缺點是因為數(shù)據(jù)修改需要同時寫入存儲，數(shù)據(jù)寫入速度較慢。
Write-back（回寫模式）在數(shù)據(jù)更新時只寫入緩存Cache。只在數(shù)據(jù)被替換出緩存時，被修改的緩存數(shù)據(jù)才會被寫到后端存儲。此模式的優(yōu)點是數(shù)據(jù)寫入速度快，因為不需要寫存儲；缺點是一旦更新后的數(shù)據(jù)未被寫入存儲時出現(xiàn)系統(tǒng)掉電的情況，數(shù)據(jù)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(shù)據(jù)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(shù)據(jù)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
