11.2. Understanding Virtual Disks

Red Hat Virtualization features Preallocated (thick provisioned) and Sparse (thin provisioned) storage options.
  • Preallocated
    A preallocated virtual disk allocates all the storage required for a virtual machine up front. For example, a 20 GB preallocated logical volume created for the data partition of a virtual machine will take up 20 GB of storage space immediately upon creation.
  • Sparse
    A sparse allocation allows an administrator to define the total storage to be assigned to the virtual machine, but the storage is only allocated when required.
    For example, a 20 GB thin provisioned logical volume would take up 0 GB of storage space when first created. When the operating system is installed it may take up the size of the installed file, and would continue to grow as data is added up to a maximum of 20 GB size.
You can view a virtual disk's ID in the Disks tab. The ID is used to identify a virtual disk because its device name (for example, /dev/vda0) can change, causing disk corruption. You can also view a virtual disk's ID in /dev/disk/by-id.
You can view the Virtual Size of a disk in the Disks tab and in the Disks sub-tab of the Storage, Virtual Machines, and Templates tabs. The Virtual Size is the total amount of disk space that the virtual machine can use. It is the number that you enter in the Size(GB) field when you create or edit a virtual disk.
You can view the Actual Size of a disk in the Disks sub-tab of the Storage and Templates tabs. This is the amount of disk space that has been allocated to the virtual machine so far. Preallocated disks show the same value for Virtual Size and Actual Size. Sparse disks may show different values, depending on how much disk space has been allocated.

Note

When creating a Cinder virtual disk, the format and type of the disk are handled internally by Cinder and are not managed by Red Hat Virtualization.
The possible combinations of storage types and formats are described in the following table.

Table 11.1. Permitted Storage Combinations

Storage Format Type Note
NFS or iSCSI/FCP RAW or QCOW2 Sparse or Preallocated  
NFS RAW Preallocated A file with an initial size which equals the amount of storage defined for the virtual disk, and has no formatting.
NFS RAW Sparse A file with an initial size which is close to zero, and has no formatting.
NFS QCOW2 Sparse A file with an initial size which is close to zero, and has QCOW2 formatting. Subsequent layers will be QCOW2 formatted.
SAN RAW Preallocated A block device with an initial size which equals the amount of storage defined for the virtual disk, and has no formatting.
SAN QCOW2 Sparse A block device with an initial size which is much smaller than the size defined for the virtual disk (currently 1 GB), and has QCOW2 formatting for which space is allocated as needed (currently in 1 GB increments).