code: func (d *Driver) setStorageSize(dir string, driver *Driver) error {
  	if driver.options.size <= 0 {
  		return fmt.Errorf("btrfs: invalid storage size: %s", units.HumanSize(float64(driver.options.size)))
  	}
  	if d.options.minSpace > 0 && driver.options.size < d.options.minSpace {
  		return fmt.Errorf("btrfs: storage size cannot be less than %s", units.HumanSize(float64(d.options.minSpace)))
  	}
  	if err := d.subvolEnableQuota(); err != nil {
  		return err
  	}
  	return subvolLimitQgroup(dir, driver.options.size)
  }
docs: // Set btrfs storage size