在Cisco Server的DRAC中, 創建virtual drive時, 會看到下面的選項.
那么Write back, write through, write back bad BBU之間的區別是什么呢?
Cisco官網解釋如下:
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Write through
Data is written through the cache and to the physical drives. Performance is improved, because subsequent reads of that data can be satisfied from the cache.
Write back
Data is stored in the cache, and is only written to the physical drives when space in the cache is needed. Virtual drives requesting this policy fall back to Write Through caching when the BBU cannot guarantee the safety of the cache in the event of a power failure.
Write back bad BBU
With this policy, write caching remains Write Back even if the battery backup unit is defective or discharged.
Write back good BBU
LSI's term is "Write Back"
簡單點說, write back bad BBU性能好, 但不安全, 因為BBU壞了, 一旦斷點, 就會有cache里的沒被寫到盤上的數據丟失.
write through安全, 但寫性能不好, 因為要cache和盤上都要寫, 寫到盤上好了才算是寫好了, 讀性能提高因為剛寫的數據在cache里可以讀到, 然並卵.
write back性能好, 也安全, 因為bbu壞了, 他就自動變為write through.
注意, 對於SSD(固態硬盤)來說, write back是不合適的, 因為SSD的寫本來就不慢, 用write back的話, 就是說cache要寫一次, 后面SSD上還要寫一次.
做個顧名思義的版本吧:
Write back (to disk from cache when needed)
Write through (cache directly to disk and make a copy to cache)
Write back (to disk from cache even when) bad BBU (happens)
Write back (to disk from cache only when)good BBU (is there)
參考資料
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Cisco UCS C-Series Servers Integrated Management Controller GUI Configuration Guide, Release 1.5
LSI
http://wiki.mikejung.biz/LSI#LSI_Write_Caching_Configuration
注: 之前不仔細, 弄串了write through和write back的順序, 不過好在列出了原文, 看了覺得混亂的應該會看看原文的吧. 希望大家原諒.