1 What you are calling the OS disks is generally called the root aggregate. By default, it uses 3 disks in RAID-DP unless advanced drive partitioning is in use, in which case it uses a small partition from several disks to form the root aggregate. The OS actually boots off embedded flash media, but needs a root volume on a root aggregate to store persistent configuration and logging data. The 5 disk minimum is for a data aggregate.
2. It is possible to add disks to the root aggregate, but in practice it would be unusual to do so. Historically this was fairly common in 7mode.
3. It is possible, but not recommended, to use raid4. You can convert an aggregate from RAID-DP to RAID4 as long as it does not exceed the limits for RAID4.
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