When we choose 2 disks to create a disk group with NORMAL redundancy during Oracle grid infrastructure 19c installation, we got INS-30510: Insuffient number of ASM disks selected.
INS-30510 means that two selected disks for creating disk group are not enough in number for this type of redundancy. In other words, 2 selected ASM disks are not enough for NORMAL redundancy.
It sounds really weird, because we know it's typically a two-way mirroring for NORMAL redundancy. Is it because ASM knows the disk group is going to store cluster files?
Let's see what Oracle says about Storing Voting Files on Oracle ASM:
Normal redundancy: A normal redundancy disk group requires a minimum of two disk devices (or two failure groups). The effective disk space in a NORMAL redundancy disk group is half the sum of the disk space in all of its devices.
For Oracle Clusterware files, a NORMAL redundancy disk group requires a minimum of three disk devices (two of the three disks are used by failure groups and all three disks are used by the quorum failure group) and provides three voting files and one OCR and mirror of the OCR. When using a NORMAL redundancy disk group, the cluster can survive the loss of one failure group.
1. Use 3 Disks for NORMAL redundancy
That is to say, we need at least 3 disks for disk group containing OCR and voting files with normal redundancy.
2. Use 1 Disk for EXTERNAL redundancy
For a RAC environment with reliable storage devices, you may choose EXTERNAL redundancy as a solution if you don't have extra disks.