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

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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.

• 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.

• 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.

• 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.