The VirtualSize field is deprecated and the upcoming API version v1.44 will no longer propagate the field. See:1261fe69a3, Given that in docker 1.10 and up (API v1.22), the VirtualSize and Size fields contain the same value, and the "df" endpoint was not supported until API v1.25, we can "safely" use Size instead; see: -4ae7176ffb-4352da7803Signed-off-by: Sebastiaan van Stijn <github@gone.nl>