feat: introduce pool-level rebalance (#15483)

cmd/erasure-server-pool-decom.go

@@ -499,6 +499,15 @@ const (
 // Init() initializes pools and saves additional information about them
 // in 'pool.bin', this is eventually used for decommissioning the pool.
 func (z *erasureServerPools) Init(ctx context.Context) error {
+	// Load rebalance metadata if present
+	err := z.loadRebalanceMeta(ctx)
+	if err != nil {
+		return fmt.Errorf("failed to load rebalance data: %w", err)
+	}
+
+	// Start rebalance routine
+	z.StartRebalance()
+
 	meta := poolMeta{}
 
 	if err := meta.load(ctx, z.serverPools[0], z.serverPools); err != nil {
@@ -573,6 +582,19 @@ func (z *erasureServerPools) Init(ctx context.Context) error {
 	return nil
 }
 
+func (z *erasureServerPools) IsDecommissionRunning() bool {
+	z.poolMetaMutex.RLock()
+	defer z.poolMetaMutex.RUnlock()
+	meta := z.poolMeta
+	for _, pool := range meta.Pools {
+		if pool.Decommission != nil {
+			return true
+		}
+	}
+
+	return false
+}
+
 func (z *erasureServerPools) decommissionObject(ctx context.Context, bucket string, gr *GetObjectReader) (err error) {
 	objInfo := gr.ObjInfo