Host上storage的write policy

在Cisco Server的DRAC中, 创建irtual 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 back

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 through

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.

简单点说, write back bad BBU性能好, 但不安全, 因为BBU坏了, 一旦断点, 就会有数据丢失.

write through安全, 但写性能不好, 因为要写到盘上才算是写好了.

write back性能好, 也安全, 因为bbu坏了, 他就自动变为write through.

参考资料

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Cisco UCS C-Series Servers Integrated Management Controller GUI Configuration Guide, Release 1.5

http://www.cisco.com/en/US/docs/unified_computing/ucs/c/sw/gui/config/guide/1.5/b_Cisco_UCS_C-series_GUI_Configuration_Guide.151_chapter_01001.html

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