ASM与RAID中的条带化及镜像

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.
不能简单通过一句话来回答这个问题，因为答案取决于业务需求及预算(成本，可用性，性能及应用)，下面是一些权衡点，通过它们去对比硬体镜像条带(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条带化各有优势，相互补充。 当SAN或磁盘阵列提供条带化时，在某种程度上来讲，可以作为ASM的一种补充。

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.

理论上，Oracle建议，在SAN层的RAID条带尺寸应该和ASM条带尺寸匹配(默认为1M). 但是如果不能匹配 (即在storage层1MB条带)，那么stripe 尺寸大小是256K/128K/512k也是可以的，目前ASM 1M的条带尺寸是硬件条带尺寸的数倍，I/O在硬件层是均衡的。否则，单个I/O可能被分配到多个磁盘上，导致多次读写和过多的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 在那些实现storage控制器上外部硬件mirror功能的server上会有少量负荷(特别是写性能上)，使用外部镜像，你需要预留磁盘作为hot spares, 使用ASM,  hot spare是不需要的，因此，storage有更好的存储空间使用率。 因为failure groups,  ASM降低误配置和人为失误的几率，如果使用外部RAID, 你不得不仔细规划你的冗余控制器和路径，这些都需要更高的管理成本 。

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