參考http://en.wikipedia.org/wiki/Cache#Writing_Policies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。

http://witmax.cn/cache-writing-policies.html