code: func (d *Driver) CreateReadWrite(id, parent string, opts *graphdriver.CreateOpts) error {
  	quotaSize := d.getQuotaOpt()
  
  	if opts != nil {
  		for key, val := range opts.StorageOpt {
  			switch key {
  			case "size":
  				if !d.quotaSupported() {
  					return quota.ErrQuotaNotSupported
  				}
  				size, err := units.RAMInBytes(val)
  				if err != nil {
  					return errdefs.InvalidParameter(err)
  				}
  				quotaSize = uint64(size)
  			default:
  				return errdefs.InvalidParameter(errors.Errorf("Storage opt %s not supported", key))
  			}
  		}
  	}
  
  	return d.create(id, parent, quotaSize)
  }
docs: // CreateReadWrite creates a layer that is writable for use as a container
  // file system.