Volume

A volume provides storages for VM instances. A volume can either be a root volume or a data volume.
  • Root volume: A system disk where the VM instance operating system is installed.
  • Data volume: A data disk that provides additional storages for a VM instance.

Data volumes are mainly involved in the volume management.

Precautions

When you use volumes, note that:
  • Volumes are hypervisor specific. That is, a volume that has been attached to a VM instance of one hypervisor type cannot be attached to a VM instance of another hypervisor type. For example, a volume of KVM VM instances cannot be attached to VMware VM instances.
  • A volume can have two sizes: real size and virtual size. The real size is the size that a volume actually occupies in the storage system, while the virtual size is the size that a volume claims for. The virtual size is usually greater than or equal to the real size. As the number of written files increases, the real size will gradually increase.
  • A volume (excluding shared volumes) can only be attached to one VM instance at any given time. Ceph and Shared Block primary storages support shared volumes. A shared volume can be identified and accessed by multiple VM instances at the same time.
  • A root volume is always attached to its owner VM instance and cannot be detached.
  • A data volume can be attached to or detached from different VM instances of the same hypervisor type.
  • In the environment where multiple primary storages are available, you can specify a primary storage to create a volume. If no primary storage is specified, the default creation method is as follows:
    • For local primary storages, volumes are created from the primary storage with large capacity.
    • For NFS primary storages, volumes are created from a random primary storage.
    • For mixed primary storages (local + NFS/Shared Mount Point), volumes are created from the primary storage where the root volume of the volume does not locate.
  • You can set QoS for data volumes to limit the disk bandwidth. Note that excessive low QoS might cause low I/O performance.