11.2. Understanding Virtual Disks
- PreallocatedA 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.
- SparseA 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.
/dev/vda0) can change, causing disk corruption. You can also view a virtual disk's ID in
Table 11.1. Permitted Storage Combinations
|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).|