A volume provides storage to a guest VM. The volume can provide for a root disk or an additional data disk. CloudStack supports additional volumes for guest VMs.

Volumes are created for a specific hypervisor type. A volume that has been attached to guest using one hypervisor type (e.g, XenServer) may not be attached to a guest that is using another hypervisor type (e.g. vSphere, KVM). This is because the different hypervisors use different disk image formats.

CloudStack defines a volume as a unit of storage available to a guest VM. Volumes are either root disks or data disks. The root disk has "/" in the file system and is usually the boot device. Data disks provide for additional storage (e.g. As "/opt" or "D:"). Every guest VM has a root disk, and VMs can also optionally have a data disk. End users can mount multiple data disks to guest VMs. Users choose data disks from the disk offerings created by administrators. The user can create a template from a volume as well; this is the standard procedure for private template creation. Volumes are hypervisor-specific: a volume from one hypervisor type may not be used on a guest of another hypervisor type.