add more for volume placement

fix possible nil volume cases

weed-fs/src/pkg/replication/volume_growth.go

@@ -0,0 +1,29 @@
+package replication
+
+import (
+	"pkg/topology"
+)
+
+/*
+This package is created to resolve these replica placement issues:
+1. growth factor for each replica level, e.g., add 10 volumes for 1 copy, 20 volumes for 2 copies, 30 volumes for 3 copies
+2. in time of tight storage, how to reduce replica level
+3. optimizing for hot data on faster disk, cold data on cheaper storage,
+4. volume allocation for each bucket
+*/
+
+type VolumeGrowth struct {
+	copy1factor int
+	copy2factor int
+	copy3factor int
+	copyAll     int
+}
+
+func (vg *VolumeGrowth) GrowVolumeCopy(copyLevel int, topo topology.Topology) {
+	if copyLevel == 1 {
+		for i := 0; i <vg.copy1factor; i++ {
+			topo.RandomlyCreateOneVolume()
+		}
+	}
+
+}