User guide

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What storage will be used for the virtual machines?
Network usage
As opposed to ordinary backups (TIB files), virtual machine files are transferred uncompressed
through the network. Therefore, using a SAN or a storage local to the machine that performs
conversion is the best choice from the network usage standpoint. A local disk is not an option
though, if the conversion is performed by the same machine that is backed up. Using a NAS also
makes good sense.
Storage space
For VMware, Hyper-V and Virtual PC, disks of the resulting virtual machine will use as much storage
space as the original data occupies. Assuming that the original disk size is 100 GB and the disk stores
10 GB of data, the corresponding virtual disk will occupy about 10 GB. VMware calls this format "thin
provisioning", Microsoft uses the "dynamically expanding disk" term. Since the space is not pre-
allocated, the physical storage is expected to have sufficient free space for the virtual disks to
increase in size.
For KVM or RHEV, disks of the resulting virtual machine will have the raw format. This means that
virtual disk size is always equal to the original disk capacity. Assuming that the original disk size is 100
GB, the corresponding virtual disk will occupy 100 GB even if the disk stores 10 GB of data.
6.2.2.4 How regular conversion to VM works
The way the repeated conversions work depends on where you choose to create the virtual machine.
 If you choose to save the virtual machine as a set of files: each conversion re-creates the virtual
machine from scratch.
 If you choose to create the virtual machine on a virtualization server: when converting an
incremental or differential backup, the software updates the existing virtual machine instead of
re-creating it. Such conversion is normally faster. It saves network traffic and CPU resource of the
host that performs the conversion. If updating the virtual machine is not possible, the software
re-creates it from scratch.
The following is a detailed description of both cases.
If you choose to save the virtual machine as a set of files
As a result of the first conversion, a new virtual machine will be created. Every subsequent
conversion will re-create this machine from scratch. First, a new (temporary) virtual machine is
created. If this operation succeeds, the old machine is replaced. If an error occurs during creation of
the temporary machine, the temporary machine is deleted. This way, the conversion always ends up
with a single machine. However, extra storage space is required during conversion to store the
temporary machine.
If you choose to create the virtual machine on a virtualization server
The first conversion creates a new virtual machine. Any subsequent conversion works as follows:
 If there has been a full backup since the last conversion, the virtual machine is re-created from
scratch. This involves creating a temporary virtual machine, as described earlier in this section.
 Otherwise, the existing virtual machine is updated to reflect changes since the last conversion. If
updating is not possible (for example, if you deleted the intermediate snapshots, see below), the
virtual machine is re-created from scratch.