add --disable-scanner to backup command

The scanner process has only cosmetic effect for the progress printer,
and can be disabled without impacting functionality when the user does
not need an estimate of completion.

In many cases the scanner process can provide beneficial priming of
the file system cache, so as general advice it should not be disabled.
However, tests have shown that backup of NFS and fuse based filesystems,
where stat(2) is relatively expensive, can be significantly faster
without the scanner.

cmd/restic/cmd_backup.go

@@ -99,6 +99,7 @@ type BackupOptions struct {
 	UseFsSnapshot     bool
 	DryRun            bool
 	ReadConcurrency   uint
+	NoScan            bool
 }
 
 var backupOptions BackupOptions
@@ -138,6 +139,7 @@ func init() {
 	f.BoolVar(&backupOptions.IgnoreInode, "ignore-inode", false, "ignore inode number changes when checking for modified files")
 	f.BoolVar(&backupOptions.IgnoreCtime, "ignore-ctime", false, "ignore ctime changes when checking for modified files")
 	f.BoolVarP(&backupOptions.DryRun, "dry-run", "n", false, "do not upload or write any data, just show what would be done")
+	f.BoolVar(&backupOptions.NoScan, "no-scan", false, "do not run scanner to estimate size of backup")
 	if runtime.GOOS == "windows" {
 		f.BoolVar(&backupOptions.UseFsSnapshot, "use-fs-snapshot", false, "use filesystem snapshot where possible (currently only Windows VSS)")
 	}
@@ -586,16 +588,18 @@ func runBackup(ctx context.Context, opts BackupOptions, gopts GlobalOptions, ter
 		targets = []string{filename}
 	}
 
-	sc := archiver.NewScanner(targetFS)
-	sc.SelectByName = selectByNameFilter
-	sc.Select = selectFilter
-	sc.Error = progressPrinter.ScannerError
-	sc.Result = progressReporter.ReportTotal
+	if !opts.NoScan {
+		sc := archiver.NewScanner(targetFS)
+		sc.SelectByName = selectByNameFilter
+		sc.Select = selectFilter
+		sc.Error = progressPrinter.ScannerError
+		sc.Result = progressReporter.ReportTotal
 
-	if !gopts.JSON {
-		progressPrinter.V("start scan on %v", targets)
+		if !gopts.JSON {
+			progressPrinter.V("start scan on %v", targets)
+		}
+		wg.Go(func() error { return sc.Scan(cancelCtx, targets) })
 	}
-	wg.Go(func() error { return sc.Scan(cancelCtx, targets) })
 
 	arch := archiver.New(repo, targetFS, archiver.Options{ReadConcurrency: backupOptions.ReadConcurrency})
 	arch.SelectByName = selectByNameFilter