All the drives (root or extra data disk) connected to guest VMs are provided by the primary storage as volumes. Volumes can be requested by users from the interfaces and can be attached to any guest dynamically (except when you are using Oracle VM).

The volumes are attached to any guest with the same type of hypervisor and may not be attached to any other guest of any other hypervisor type in any other cluster because different hypervisors have different disk image formats. These volumes are the units of storage in CloudStack that are available to guests. These volumes can be used as root drives by guests as "/" in the filesystem, which is the boot device or it can be optionally mounted as an additional disk volume such as "D:" or "/opt". ...

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