feat(restore): support --transfers=0 for max speed restore and make i…

…t default

Our experiments showed that the fastest file download
was achieved for transfers=2*cpu_cnt. In order to make
it easier for the user to use, a new, special value
of --transfers=0 flag will take care of that.
It is set as the default, because we aim to make
not configured restore as fast as possible.

docs/source/sctool/partials/sctool_restore.yaml

@@ -157,9 +157,10 @@ options:
         Timezone of --cron and --window flag values.
         The default value is taken from this system, namely 'TZ' envvar or '/etc/localtime' file.
     - name: transfers
-      default_value: "2"
+      default_value: "0"
       usage: |
         Sets the amount of file transfers to run in parallel when downloading files from backup location to Scylla node.
+        Set to 0 for the fastest download (results in setting transfers to 2*node_shard_count).
     - name: window
       default_value: '[]'
       usage: |

docs/source/sctool/partials/sctool_restore_update.yaml

@@ -155,9 +155,10 @@ options:
         Timezone of --cron and --window flag values.
         The default value is taken from this system, namely 'TZ' envvar or '/etc/localtime' file.
     - name: transfers
-      default_value: "2"
+      default_value: "0"
       usage: |
         Sets the amount of file transfers to run in parallel when downloading files from backup location to Scylla node.
+        Set to 0 for the fastest download (results in setting transfers to 2*node_shard_count).
     - name: window
       default_value: '[]'
       usage: |

pkg/command/restore/cmd.go

@@ -80,7 +80,7 @@ func (cmd *command) init() {
 	w.Unwrap().StringVarP(&cmd.snapshotTag, "snapshot-tag", "T", "", "")
 	w.Unwrap().IntVar(&cmd.batchSize, "batch-size", 2, "")
 	w.Unwrap().IntVar(&cmd.parallel, "parallel", 1, "")
-	w.Unwrap().IntVar(&cmd.transfers, "transfers", 2, "")
+	w.Unwrap().IntVar(&cmd.transfers, "transfers", 0, "")
 	w.Unwrap().BoolVar(&cmd.allowCompaction, "allow-compaction", false, "")
 	w.Unwrap().BoolVar(&cmd.restoreSchema, "restore-schema", false, "")
 	w.Unwrap().BoolVar(&cmd.restoreTables, "restore-tables", false, "")

pkg/command/restore/res.yaml

@@ -35,6 +35,7 @@ parallel: |
 
 transfers: |
   Sets the amount of file transfers to run in parallel when downloading files from backup location to Scylla node.
+  Set to 0 for the fastest download (results in setting transfers to 2*node_shard_count).
 
 allow-compaction: |
   Defines if auto compactions should be running on Scylla nodes during restore.

pkg/service/restore/model.go

@@ -38,13 +38,16 @@ type Target struct {
 	locationHosts map[Location][]string `json:"-"`
 }
 
-const maxBatchSize = 0
+const (
+	maxBatchSize = 0
+	maxTransfers
+)
 
 func defaultTarget() Target {
 	return Target{
 		BatchSize: 2,
 		Parallel:  0,
-		Transfers: 2,
+		Transfers: 0,
 		Continue:  true,
 	}
 }
@@ -64,8 +67,8 @@ func (t Target) validateProperties() error {
 	if t.Parallel < 0 {
 		return errors.New("parallel param has to be greater or equal to zero")
 	}
-	if t.Transfers < 1 {
-		return errors.New("transfers param has to be greater than zero")
+	if t.Transfers < 0 {
+		return errors.New("transfers param has to be greater or equal to zero")
 	}
 	if t.RestoreSchema == t.RestoreTables {
 		return errors.New("choose EXACTLY ONE restore type ('--restore-schema' or '--restore-tables' flag)")

pkg/service/restore/tables_worker.go

@@ -200,8 +200,12 @@ func (w *tablesWorker) stageRestoreData(ctx context.Context) error {
 	}
 
 	for _, h := range hosts {
-		if err := w.client.RcloneSetTransfers(ctx, h, w.target.Transfers); err != nil {
-			return errors.Wrapf(err, "set transfers on %s to %d", h, w.target.Transfers)
+		transfers := w.target.Transfers
+		if transfers == maxTransfers {
+			transfers = 2 * int(hostToShard[h])
+		}
+		if err := w.client.RcloneSetTransfers(ctx, h, transfers); err != nil {
+			return errors.Wrapf(err, "set transfers on %s to %d", h, transfers)
 		}
 	}