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ASM knows about Oracle database files and uses different and optimal stripping sizes based on file templates. ASM uses file extents (AU units) distributed on available free space of disk failure groups. RAID mirrors or stripes disks and does not know anything about Oracle Database. ASM provides more configuration and space management flexibility. For instance you can add disks without having to rebuild your configuration. You cannot do that with RAID.
From a performance, redundancy and space utilization aspect, multiple ASM failure groups (single or multiple disks) can provide a better value for money than than RAID when using Oracle Database products. You can build ASM disk groups that using disk failure groups that are RAID devices and gain additional performance and disk level redundancy, but you can also configure ASM disk groups to rely on external redundancy (RAID) if you wish.
ASM is standard and recommended to store Oracle database files. ASM is not a substitute or software solution for RAID. It's not RAID at all. RAID and ASM are complementary solutions and building ASM using RAID devices provides additional benefits.
You should do some reading on Storage technologies as well as ASM. While ASM can live on discrete devices (JBODS), you can also have storage arrays that can have RAIDed devices with ASM LUNS carved out of that storage. Every storage vendor will do these functions differently and you, as a DBA (especially a RAC DBA) should be familiar with all of these technologies. If you understand the concepts, being able support those environments will be a lot easier.
Look for the book "Oracle Automatic Storage Management: Under-the-Hood & Practical Deployment Guide (Oracle Press)"
ASM is NOT RAID. It does not act like RAID. You can use a RAIDed set of DEVICES/LUNS as ASM devices. But, keep in mind, these devices should not be expanded and ALL ASM devices in a single diskgroup MUST be the same size. And do NOT make the mistake of thinking that partitioning a DEVICE to equal sizes is acceptable. Some OS's require that you partition the device starting at cylinder 2 through last and use this device (UNIX) - with Windows, you are on your own to figure out...
Disks in a given disk group should have similar size and performance characteristics, but its not a must.
You can use different size disks, but when a disks runs full, ASM needs to rebalance data among the remaining free space. There might be situations where it is acceptable to temporarily have disks of different sizes and performance characteristics coexist in a disk group. This would be the case when migrating from an old set of disks to a new set of disks.