ASM與RAID中的條帶化及映象

Note:330398.1   

 

How does ASM work with RAID where striping and mirroring are already built-in?

There is really no single answer to this question since it depends on business requirement and budget (cost, availability, performance & utilization). The followings are some trade offs that needs to be considered to compare hardware mirroring-stripping and ASM.
不能簡單透過一句話來回答這個問題，因為答案取決於業務需求及預算(成本，可用性，效能及應用)，下面是一些權衡點，透過它們去對比硬體映象條帶(RAID)和ASM . 

 

 

ASM & RAID striping are complimentary to each other. When a SAN or disk array provides striping, that can be used in a manner which is complementary to ASM.

ASM和RAID條帶化各有優勢，相互補充。 當SAN或磁碟陣列提供條帶化時，在某種程度上來講，可以作為ASM的一種補充。

 

 

Oracle ideally suggest that the RAID stripe size at the SAN layer should match ASM stripe size (1MB by default). However, if the above is not possible (1MB stripe at storage level),then a stripe size of 256K/128K/512k should be ok. As long ASM 1MB stripe size is a multiple of hardware stripe size, I/O is aligned at hardware level. Otherwise, a single I/O can be split into multiple disks and cause multiple read writes and excessive i/o operations.

理論上，Oracle建議，在SAN層的RAID條帶尺寸應該和ASM條帶尺寸匹配(預設為1M). 但是如果不能匹配 (即在storage層1MB條帶)，那麼stripe 尺寸大小是256K/128K/512k也是可以的，目前ASM 1M的條帶尺寸是硬體條帶尺寸的數倍，I/O在硬體層是均衡的。否則，單個I/O可能被分配到多個磁碟上，導致多次讀寫和過多的I/O操作。

 

 

ASM mirroring has a small overhead on the server (specially on write performance) where external hardware mirroring performs the function on the storage controller.With external mirroring, you need to reserve disks as hot spares.  With ASM, hot spares are not necessary and therefore, more efficient use of the storage capacity. ASM reduces the chance of mis-configuation and human error because of failure groups. With external RAID, you have to carefully plan your redundant controllers and paths which requires higher admin overhead. 

 

ASM mirroring 在那些實現storage控制器上外部硬體mirror功能的server上會有少量負荷(特別是寫效能上)，使用外部映象，你需要預留磁碟作為hot spares, 使用ASM,  hot spare是不需要的，因此，storage有更好的儲存空間使用率。 因為failure groups,  ASM降低誤配置和人為失誤的機率，如果使用外部RAID, 你不得不仔細規劃你的冗餘控制器和路徑，這些都需要更高的管理成本 。