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RAID groups and parity groups

We use a HDS Thunder 9200 SAN configured at the back end with two RAID-1 groups (2 hard disks in each group). However,...

I can actually see three LUNs at the OS level. I don't quite understand, because I think each RAID group should be a LUN by itself. In this instance, one of the RAID groups obviously consists of two LUNs. Is that possible?

Yup. The Thunder 9200 supports the notion of parity groups and RAID groups. RAID groups can contain single or multiple parity groups. You can think of the RAID group as the actual RAID container for data protection, and the parity group as a partition of that container. This way, multiple LUNs can be created from each RAID group, and ported out to the same or different servers. This allows granularity in LUN sizes being obtained from the RAID group.

If each partition (parity group) is assigned to the same server, there should be no contention for the RAID group's disk resources. You can always just use the entire RAID group as a single parity group and create one big LUN.

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