定義
對于某個(gè)存儲(chǔ)對象（file/block）在一個(gè)時(shí)間點(diǎn)的數(shù)據(jù)拷貝
Snapshot – A point-in-time copy of data that is stored on the storage system.
業(yè)界針對snapshot兩種的通用技術(shù)
Copy-on-Write
寫時(shí)拷貝：在對snapshot觉鼻，或者有snapshot的存儲(chǔ)對象進(jìn)程復(fù)寫之前妇智，先將對象之前的數(shù)據(jù)拷貝到一個(gè)制定的地方敢朱，用于保存那些snapped的數(shù)據(jù)
in order to preserve the snapped data, copy blocks to a designated protection space prior to being overwritten
Redirect on Write 寫重定向
在對snapshot，或者有snapshot的存儲(chǔ)對象進(jìn)程復(fù)寫時(shí)鸥咖，將新的寫入落盤到其他地方，同時(shí)更新對應(yīng)的指針指辽慕。
With redirect on write, new writes to snapped storage resources or their snapshots are redirected to a new location in the same pool, and pointers are updated to point to the new location.
兩種技術(shù)的對比：
結(jié)論： 寫重定向效率更優(yōu)蜡塌，原因如下：
1、“寫重定向”不需要設(shè)置專門的空間用來存儲(chǔ)那些snapped的數(shù)據(jù)适室，而“寫時(shí)拷貝”方案則需要
2嫡意、“寫重定向”不需要拷貝轉(zhuǎn)移那些snapped的數(shù)據(jù)，而“寫時(shí)拷貝”方案則需要
