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ASM Misconceptions

There are some misconceptions with modern ASM. One huge one is mirroring is required.

Per Doc ID 810394.1, "Use Oracle ASM mirroring redundancy when not using hardware RAID, or when you need host-based volume management functionality, such as mirroring across storage systems".

Otherwise you can rely on your hardware SAN for mirroring. This both simplifies your ASM architecture and provides the optimal performance to ASM as a RAID type solution.

Here is what the 19c Oracle docs say on this:
Oracle ASM mirroring runs on the database server and Oracle recommends to off load this processing to the storage hardware RAID controller by using external redundancy .
Source of above here

Another misconception is that ASM mirroring will help you survive all disk issues. Nope! Do not rely on ASM exclusively for this. I have seen instances first hand where a vendor changed OS level properties and ALL the ASM disks were instantly corrupted requiring a complete RAC re-installation. Luckily I had a recent full database export and RMAN backup. ;-)

ASM was created in large part to deal with inferior hardware. If you have a state-of-the-art SAN then use the External disk option for ASM in 12c and later.

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