create new volumes on less occupied disk locations (#7349)

* create new volumes on less occupied disk locations

* add unit tests

* address comments

* fixes

weed/storage/store_load_balancing_simple_test.go

@@ -0,0 +1,51 @@
+package storage
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+// TestLoadBalancingDistribution tests that volumes are evenly distributed
+func TestLoadBalancingDistribution(t *testing.T) {
+	// Create test store with 3 directories
+	store := newTestStore(t, 3)
+
+	// Create 9 volumes and verify they're evenly distributed
+	volumesToCreate := 9
+	for i := 1; i <= volumesToCreate; i++ {
+		volumeId := needle.VolumeId(i)
+
+		err := store.AddVolume(volumeId, "", NeedleMapInMemory, "000", "",
+			0, needle.GetCurrentVersion(), 0, types.HardDriveType, 3)
+
+		if err != nil {
+			t.Fatalf("Failed to add volume %d: %v", volumeId, err)
+		}
+	}
+
+	// Check distribution - should be 3 volumes per location
+	for i, location := range store.Locations {
+		localCount := location.LocalVolumesLen()
+		if localCount != 3 {
+			t.Errorf("Location %d: expected 3 local volumes, got %d", i, localCount)
+		}
+	}
+
+	// Verify specific distribution pattern
+	expected := map[int][]needle.VolumeId{
+		0: {1, 4, 7},
+		1: {2, 5, 8},
+		2: {3, 6, 9},
+	}
+
+	for locIdx, expectedVols := range expected {
+		location := store.Locations[locIdx]
+		for _, vid := range expectedVols {
+			if _, found := location.FindVolume(vid); !found {
+				t.Errorf("Location %d: expected to find volume %d, but it's not there", locIdx, vid)
+			}
+		}
+	}
+}