服务器主机上RAID Card的Write Caching Policy
在Cisco Server的DRAC中, 创建virtual drive时, 会看到下面的选项.
那么Write back, write through, write back bad BBU之间的区别是什么呢?
Cisco官网解释如下:
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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"
简单点说, write back bad BBU性能好, 但不安全, 因为BBU坏了, 一旦断点, 就会有cache里的没被写到盘上的数据丢失.
write through安全, 但写性能不好, 因为要cache和盘上都要写, 写到盘上好了才算是写好了, 读性能提高因为刚写的数据在cache里可以读到, 然并卵.
write back性能好, 也安全, 因为bbu坏了, 他就自动变为write through.
注意, 对于SSD(固态硬盘)来说, write back是不合适的, 因为SSD的写本来就不慢, 用write back的话, 就是说cache要写一次, 后面SSD上还要写一次.
做个顾名思义的版本吧:
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)
参考资料
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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: 之前不仔细, 弄串了write through和write back的顺序, 不过好在列出了原文, 看了觉得混乱的应该会看看原文的吧. 希望大家原谅.
