Add --buffer-size parameter to control buffer size for copy

docs/content/docs.md

@@ -261,6 +261,13 @@ you have a 10 Mbit/s connection and you wish rclone to use half of it
 - 5 Mbit/s.  This is 5/8 = 0.625MByte/s so you would use a `--bwlimit
 0.625M` parameter for rclone.
 
+### --buffer-size=SIZE ###
+
+Use this sized buffer to speed up file transfers.  Each `--transfer`
+will use this much memory for buffering.
+
+Set to 0 to disable the buffering for the minimum memory use.
+
 ### --checkers=N ###
 
 The number of checkers to run in parallel.  Checkers do the equality

fs/accounting.go

@@ -348,11 +348,15 @@ func NewAccount(in io.ReadCloser, obj Object) *Account {
 //
 // If the file is above a certain size it adds an Async reader
 func NewAccountSizeNameWithBuffer(in io.ReadCloser, size int64, name string) *Account {
+	const bufSize = 128 * 1024
+	var buffers int
+	if size >= int64(Config.BufferSize) {
+		buffers = int(int64(Config.BufferSize) / bufSize)
+	} else {
+		buffers = int(size / bufSize)
+	}
 	// On big files add a buffer
-	if size > 10<<20 {
-		const memUsed = 16 * 1024 * 1024
-		const bufSize = 128 * 1024
-		const buffers = memUsed / bufSize
+	if buffers > 0 {
 		newIn, err := newAsyncReader(in, buffers, bufSize)
 		if err != nil {
 			Errorf(name, "Failed to make buffer: %v", err)

fs/config.go

@@ -91,6 +91,7 @@ var (
 	backupDir       = StringP("backup-dir", "", "", "Make backups into hierarchy based in DIR.")
 	suffix          = StringP("suffix", "", "", "Suffix for use with --backup-dir.")
 	bwLimit         BwTimetable
+	bufferSize      SizeSuffix = 16 << 20
 
 	// Key to use for password en/decryption.
 	// When nil, no encryption will be used for saving.
@@ -99,6 +100,7 @@ var (
 
 func init() {
 	VarP(&bwLimit, "bwlimit", "", "Bandwidth limit in kBytes/s, or use suffix b|k|M|G or a full timetable.")
+	VarP(&bufferSize, "buffer-size", "", "Buffer size when copying files.")
 }
 
 // crypt internals
@@ -213,6 +215,7 @@ type ConfigInfo struct {
 	DataRateUnit       string
 	BackupDir          string
 	Suffix             string
+	BufferSize         SizeSuffix
 }
 
 // Return the path to the configuration file
@@ -358,6 +361,7 @@ func LoadConfig() {
 	Config.NoUpdateModTime = *noUpdateModTime
 	Config.BackupDir = *backupDir
 	Config.Suffix = *suffix
+	Config.BufferSize = bufferSize
 
 	ConfigPath = *configFile