diff --git a/go/vt/mysqlctl/backupstorage/interface.go b/go/vt/mysqlctl/backupstorage/interface.go
index 03e7b64949c..e4c2e6bc18d 100644
--- a/go/vt/mysqlctl/backupstorage/interface.go
+++ b/go/vt/mysqlctl/backupstorage/interface.go
@@ -30,6 +30,10 @@ var (
 	// BackupStorageImplementation is the implementation to use
 	// for BackupStorage. Exported for test purposes.
 	BackupStorageImplementation = flag.String("backup_storage_implementation", "", "which implementation to use for the backup storage feature")
+	// FileSizeUnknown is a special value indicating that the file size is not known.
+	// This is typically used while creating a file programmatically, where it is
+	// impossible to compute the final size on disk ahead of time.
+	FileSizeUnknown = int64(-1)
 )
 
 // BackupHandle describes an individual backup.
@@ -50,7 +54,9 @@ type BackupHandle interface {
 	// The context is valid for the duration of the writes, until the
 	// WriteCloser is closed.
 	// filesize should not be treated as an exact value but rather
-	// as an approximate value
+	// as an approximate value.
+	// A filesize of -1 should be treated as a special value indicating that
+	// the file size is unknown.
 	AddFile(ctx context.Context, filename string, filesize int64) (io.WriteCloser, error)
 
 	// EndBackup stops and closes a backup. The contents should be kept.
diff --git a/go/vt/mysqlctl/builtinbackupengine.go b/go/vt/mysqlctl/builtinbackupengine.go
index 5f8af216dc0..f49f8fdf780 100644
--- a/go/vt/mysqlctl/builtinbackupengine.go
+++ b/go/vt/mysqlctl/builtinbackupengine.go
@@ -303,7 +303,7 @@ func (be *BuiltinBackupEngine) backupFiles(ctx context.Context, params BackupPar
 	}
 
 	// open the MANIFEST
-	wc, err := bh.AddFile(ctx, backupManifestFileName, 0)
+	wc, err := bh.AddFile(ctx, backupManifestFileName, backupstorage.FileSizeUnknown)
 	if err != nil {
 		return vterrors.Wrapf(err, "cannot add %v to backup", backupManifestFileName)
 	}
diff --git a/go/vt/mysqlctl/cephbackupstorage/ceph.go b/go/vt/mysqlctl/cephbackupstorage/ceph.go
index 29b76e1b15e..80c37ceb828 100644
--- a/go/vt/mysqlctl/cephbackupstorage/ceph.go
+++ b/go/vt/mysqlctl/cephbackupstorage/ceph.go
@@ -88,7 +88,8 @@ func (bh *CephBackupHandle) AddFile(ctx context.Context, filename string, filesi
 
 		// Give PutObject() the read end of the pipe.
 		object := objName(bh.dir, bh.name, filename)
-		_, err := bh.client.PutObjectWithContext(ctx, bucket, object, reader, -1, minio.PutObjectOptions{ContentType: "application/octet-stream"})
+		// If filesize is unknown, the caller should pass in -1 and we will pass it through.
+		_, err := bh.client.PutObjectWithContext(ctx, bucket, object, reader, filesize, minio.PutObjectOptions{ContentType: "application/octet-stream"})
 		if err != nil {
 			// Signal the writer that an error occurred, in case it's not done writing yet.
 			reader.CloseWithError(err)
diff --git a/go/vt/mysqlctl/xtrabackupengine.go b/go/vt/mysqlctl/xtrabackupengine.go
index fc9e27e2523..08d18638bce 100644
--- a/go/vt/mysqlctl/xtrabackupengine.go
+++ b/go/vt/mysqlctl/xtrabackupengine.go
@@ -158,7 +158,7 @@ func (be *XtrabackupEngine) ExecuteBackup(ctx context.Context, params BackupPara
 
 	// open the MANIFEST
 	params.Logger.Infof("Writing backup MANIFEST")
-	mwc, err := bh.AddFile(ctx, backupManifestFileName, 0)
+	mwc, err := bh.AddFile(ctx, backupManifestFileName, backupstorage.FileSizeUnknown)
 	if err != nil {
 		return false, vterrors.Wrapf(err, "cannot add %v to backup", backupManifestFileName)
 	}