If you’re running virturalbox 4 on ext4 file system, while installing a virtual machine you might get a warning message says:

“The virtual machine execution may run into an error condition as described below. We suggest that you take an appropriate action to avert the error.
The host I/O cache for at least one controller is disabled and the medium ‘/home/MyUserName/.VirtualBox/HardDisks/Ubuntu_LAMP.vdi’ for this VM is located on an ext4 partition. There is a known Linux kernel bug which can lead to the corruption of the virtual disk image under these conditions.
Either enable the host I/O cache permanently in the VM settings or put the disk image and the snapshot folder onto a different file system.
The host I/O cache will now be enabled for this medium.”

This’s Linux kernel issue, and it is not necessary for normal user to concern about. Just follow what it said in the warning to enable “host I/O cache” and the issue fixed safely.