Spread each server group across a three-rack's physical racks

src/derive/rackLayout.test.ts

@@ -74,6 +74,26 @@ describe('three-rack', () => {
     expect([left.usedU, mid.usedU, right.usedU]).toEqual([15, 15, 15])
   })
 
+  it('spreads a storage group across the racks even when workers fill them unevenly', () => {
+    const plan = createEmptyPlan()
+    const rack = plan.partitions[0].racks[0]
+    rack.kind = 'three-rack'
+    // 13 worker chassis x 3U pack to 15/15/12U; without the per-group spread
+    // the emptiest rack would then take two of the three 2U storage systems.
+    rack.servers[0].count = 13 * 8
+    rack.servers.push({
+      id: 'storage',
+      role: 'storage',
+      modelId: 'server-superserver-tn12',
+      count: 3,
+      uplink: '2x25G',
+    })
+    const layout = deriveRackLayout(plan)[0]
+    for (const physical of layout.racks.slice(1)) {
+      expect(physical.slots.filter((s) => s.kind === 'storage')).toHaveLength(1)
+    }
+  })
+
   it('flags overflow when the three racks are full', () => {
     const plan = createEmptyPlan()
     const rack = plan.partitions[0].racks[0]

src/derive/rackLayout.ts

@@ -63,9 +63,11 @@ interface Item {
 /** One physical rack of a plan rack: a single rack has one (no position);
  *  a three-rack has left/mid/right, with the switches in the middle and
  *  chassis distributed evenly by used height (each chassis goes to the
- *  physical rack with the fewest used units; ties favor mid, then left,
- *  then right). A chassis that fits nowhere lands in the least-used rack,
- *  where the height check flags it. */
+ *  physical rack holding the fewest chassis of its group, the least-used
+ *  one among those; ties favor mid, then left, then right). Spreading each
+ *  group first keeps e.g. storage systems across the racks even when the
+ *  worker chassis fill them unevenly. A chassis that fits nowhere lands in
+ *  the chosen rack, where the height check flags it. */
 export interface PhysicalRack {
   position?: RackPosition
   name: string
@@ -109,8 +111,13 @@ export function physicalRacks(rack: Rack, fabric: FabricConfig): PhysicalRack[]
   const mid = { items: [] as Item[], used: switches.reduce((u, i) => u + i.units, 0) }
   const left = { items: [] as Item[], used: 0 }
   const right = { items: [] as Item[], used: 0 }
+  const chassisOfGroup = (bin: { items: Item[] }, groupId?: string) =>
+    bin.items.filter((item) => item.groupId === groupId).length
   for (const c of chassis) {
-    const target = [mid, left, right].reduce((best, r) => (r.used < best.used ? r : best))
+    const target = [mid, left, right].reduce((best, r) => {
+      const spread = chassisOfGroup(r, c.groupId) - chassisOfGroup(best, c.groupId)
+      return spread < 0 || (spread === 0 && r.used < best.used) ? r : best
+    })
     target.items.push(c)
     target.used += c.units
   }