You can configure OpenStack to write metadata to a special configuration drive that attaches to the instance when it boots. The instance can mount this drive and read files from it to get information that is normally available through the metadata service. This metadata is different from the user data.
One use case for using the configuration drive is to pass a networking configuration when you do not use DHCP to assign IP addresses to instances. For example, you might pass the IP address configuration for the instance through the configuration drive, which the instance can mount and access before you configure the network settings for the instance.
Any modern guest operating system that is capable of mounting an ISO 9660 or VFAT file system can use the configuration drive.
