A storage tier is a collection of blocks of contiguous storage, all of which have the same RAID level. This collection is spread across one or more drive groups within a given Storage Domain. Storage tiers are used by administrators and by the Oracle FS System to provision logical volumes.
For capacity HDDs, the system creates a RAID 6 storage tier, which implements double redundancy.
For all other drives, the system creates a RAID 5 storage tier, which implements single redundancy, and a RAID 10 storage tier, which implements mirroring.
The storage tier on which the system places the LUN depends on the Storage Class and on the redundancy level (single parity, double parity, or mirroring) that the administrator selects for the volume.