Working with differencing disks in VirtualBox

For some time now I have been trying to make persistent differencing disks work in VirtualBox. The idea is to have one base disk with all the software that all the VMs you want to deploy needs. Typically the OS, all patches up until the time you created the base disk, common apps like Flash etc. You then generalize this disk. In the Windows world this means running the System Preparation Tool (sysprep.exe) to remove all machine specific data from a VM. Now you can use this disk as the starting point for several new VMs and save a lot of disk space. Typically all virtualization software is able to use a differencing disk, or delta disk, for each VM. This disk only stores the changes made to the base disk in a new disk file, without making any changes to the base disk itself.

In VirtualBox the immutable disk type is what you use for the base disk. Once it is generalized you change its type from normal to immutable. Whenever you create a new VM and connect the immutable disk to it; VirtualBox will create a differencing disk with the VMs changes. This is where I have been having problems. In my scenario I want to keep the VM state in the differencing disk file between reboots, but VirtualBox, by default, will reset the differencing disk on every power cycle of the VM if the VM is running with an immutable base disk. According to the documentation you can modify this behaviour with the VBoxManage.exe tool. Here’s how:

VBoxManage.exe modifyhd <differencing disk file or disk uuid> –autoreset off

This is what I have been doing with my differencing disks. When I later inspected the disk with VBoxManage.exe, it correctly stated that autoreset was indeed off. But whenever the VM power cycled the disk was reset and the autoreset flag was set back to on.

Initially I thought that maybe this was a bug in VirtualBox regarding the VHD disk format, which is what I use for all my VMs, but the same problem exists with VirtualBox’s own virtual disk format VDI. I also read several posts from other people struggelig with this functionality, but their problems were related to the actual use of VBoxManage.exe, were for different host OSs (I use Windows 7/8), or something else. The reason I kept on trying, though, was that some people were actually able to make it work.

At some point I found another disk type in VirtualBox, the multiattach type. I really had not used it before since all the documentation on how use persistent differencing disks in VirtualBox used immutable disks. This is what the VirtualBox documentation online has to say about the multiattach disk type:

An image in multiattach mode can be attached to more than one virtual machine at the same time, even if these machines are running simultaneously. For each virtual machine to which such an image is attached, a differencing image is created. As a result, data that is written to such a virtual disk by one machine is not seen by the other machines to which the image is attached; each machine creates its own write history of the multiattach image.

Technically, a “multiattach” image behaves identically to an “immutable” image except the differencing image is not reset every time the machine starts.

So there it was! The multiattach disk type is the one to use! I changed by base disk from immutable to multiattach and now my VMs keep their differencing disks across power cycles. It seems that even though VBoxManage.exe can change the autoreset type of a differencing disk that is running off an immutable disk; the flag is not honored and is reset on every power cycle. The question now is why you can actually change that flag when it will not be honored? Would be very simple to just inform the user that immutable disks will always reset, and that multiattach should be used if you want to keep the VM state.

Anyway, VirtualBox is a great product, and also free!

Here is the link regarding the disk types in VirtualBox: http://www.virtualbox.org/manual/ch05.html#hdimagewrites

Until next time!