Backing Storage is the storage used to provide the expanded operating system image, and any ephemeral storage. Inside the virtual machine, this is normally presented as two virtual hard disks (for example, /dev/vda and /dev/vdb respectively). However, inside OpenStack, this can be derived from one of three methods: LVM, QCOW or RAW, chosen using the libvirt_images_type option in nova.conf on the compute node. QCOW is the default backing store. It uses a copy-on-write philosophy to delay allocation of storage until it is actually needed. This means that the space required for the backing of an image can be significantly less on the real disk than what seems available in the virtual machine operating system. RAW creates files without any sort of file formatting, effectively creating files with the plain binary one would normally see on a real disks. This can increase performance, but means that the entire size of the virtual disk is reserved on the physical disk. Local LVM volumes can also be used. Set libvirt_images_volume_group=nova_local where nova_local is the name of the LVM group you have created.