randomize a bit for ec shards distribution

weed/shell/command_ec_encode.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"github.com/chrislusf/seaweedfs/weed/pb"
 	"io"
+	"math/rand"
 	"sync"
 	"time"
 
@@ -248,7 +249,7 @@ func parallelCopyEcShardsFromSource(grpcDialOption grpc.DialOption, targetServer
 func balancedEcDistribution(servers []*EcNode) (allocated [][]uint32) {
 	allocated = make([][]uint32, len(servers))
 	allocatedShardIdIndex := uint32(0)
-	serverIndex := 0
+	serverIndex := rand.Intn(len(servers))
 	for allocatedShardIdIndex < erasure_coding.TotalShardsCount {
 		if servers[serverIndex].freeEcSlot > 0 {
 			allocated[serverIndex] = append(allocated[serverIndex], allocatedShardIdIndex)