add fsync option for benchmark

weed/command/benchmark.go

@@ -42,6 +42,7 @@ type BenchmarkOptions struct {
 	grpcDialOption   grpc.DialOption
 	masterClient     *wdclient.MasterClient
 	grpcRead         *bool
+	fsync            *bool
 }
 
 var (
@@ -67,6 +68,7 @@ func init() {
 	b.cpuprofile = cmdBenchmark.Flag.String("cpuprofile", "", "cpu profile output file")
 	b.maxCpu = cmdBenchmark.Flag.Int("maxCpu", 0, "maximum number of CPUs. 0 means all available CPUs")
 	b.grpcRead = cmdBenchmark.Flag.Bool("grpcRead", false, "use grpc API to read")
+	b.fsync = cmdBenchmark.Flag.Bool("fsync", false, "flush data to disk after write")
 	sharedBytes = make([]byte, 1024)
 }
 
@@ -230,6 +232,7 @@ func writeFiles(idChan chan int, fileIdLineChan chan string, s *stat) {
 			Reader:   &FakeReader{id: uint64(id), size: fileSize, random: random},
 			FileSize: fileSize,
 			MimeType: "image/bench", // prevent gzip benchmark content
+			Fsync: *b.fsync,
 		}
 		ar := &operation.VolumeAssignRequest{
 			Count:       1,