Start with four physical drives.
Use two physical drives to create pool A and two physical drives to create pool B.
Use arrays A and B to create spanned pool 1. As part of a spanned pool, pools A and B are now called "subarrays."
A sub-logical device is created in pool A and another sub-logical device is created in pool B.
Then, create a logical device in spanned pool 1.
If the sub-logical device in subarray A is in the optimal state and the sub-logical device in subarray B is in the optimal state, then the logical device in spanned pool 1 is in the optimal state.
If a physical drive fails in subarray A, then the sub-logical device in subarray A changes to the degraded state. Subarray B is still in the optimal state. The logical device in spanned pool 1 changes to the degraded state.
If the second physical drive fails in subarray A, then the sub-logical device in subarray A changes to the failed state. Subarray B remains in the optimal state. The logical device in spanned pool 1 changes to the failed state.
If the sub-logical device in subarray A is in the degraded state and a physical drive fails in subarray B, then the sub-logical device in subarray B changes to the degraded state. With both sub-logical devices in the degraded state, the logical device in spanned pool 1 remains in the degraded state.
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