Whether the controller node pairs live at the top or bottom of the set of chassis doesn't matter, as long as you: a) start with the controller node pair, b) add additional expansion chassis in one direction, c) leave no gaps between chassis, d) add new expansion chassis in the next available space beyond the farthest expansion chassis from a controller node pair.
If you want to go bottom-up or top-down, it doesn't matter much, just start with the controller node pair and pick which direction. Leave room beyond the last expansion chassis for additional expansion chassis. Don't put expansions on both top and bottom sides of a controller node pair.
The main reason for keeping everything close is because of the length of the SAS cabling between the units—they're relatively short cables. Leaving gaps between chassis would be detrimental because of the recabling efforts needed when expansions need to be installed. This is particularly bad if you're expecting to keep the array online with expansion.
For those with a 7400 or 7450 four-node configuration, they grow in both directions, so it's usually best to simply mount the controller nodes in the middle of the rack, as they grow both up and down, with each side's SAS cabling tied to a particular node pair.
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