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Important Note regarding Ext4: if your virtualbox host is Linux and your VMs are stored on XFS or Ext4 file systems, then it is absolutely necessary that you enable the "Host I/O Cache" for all virtual disk controllers after importing the OVF file, or you run the risk of file system corruption in your guest. On one of our test systems (Linux kernel 3.14.23, Virtualbox 4.3.14) the VM wouldn't even fully get through the first boot before the virtual disks got corrupted - but with Host I/O Cache on everything is fine.
Other Hypervisors
To use OVF files with other virtualisation systems like KVM you may have to convert the OVF file to a native format; Two well-known tools for this purpose are virt-v2v (available in CentOS6/RHEL) and virt-convert (in Debian).
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