Expanding VirtualBox Dynamic VDIs

There is no way to increase the maximum size of a dynamic VDI. You can, however, clone a smaller VDI to a larger one. Using this method, you can give your guest OS some more breathing room.

A commonly asked question in the VirtualBox IRC channel (#vbox on FreeNode) is some variant of “How can I make my dynamic disk image bigger?”

VirtualBox allows you to create dynamically expanding virtual disk images (VDIs) for your VMs. This means the file containing the virtual hard disk on the host only takes up as much space as the guest is actually using. When you create the dynamic VDI, the size you specify is the maximum size the VDI will expand to. This allows your guest OS to make the assumptions it needs to about drive geometry, etc.

There is no way to increase the maximum size of a dynamic VDI. You can, however, clone a smaller VDI to a larger one. Using this method, you can give your guest OS some more breathing room.

Update: As of version 4.0, VirtualBox has support for resizing dynamic VDIs and VHDs. Read the fine manual for more information. If you’re not using a VDI or a VHD (or if you’re not using a dynamic variant of one of those), read on… The technique described here should work for any image type VirtualBox supports.

Update: Windows users may want to check out mpack’s CloneVDI tool. It provides a user friendly GUI interface for a lot of VDI management tasks (including this one)

WARNING: This process will not work if your current VDI is smaller than 4GB. If your current VDI is smaller than 4GB, do not attempt this method (Update: The 4GB restriction no longer applies as of version 4.0). Also, do not delete your original VDI before you’re sure the new one works.

The basic process is as follows:

  1. Create a new (larger) VDI
  2. Clone the existing VDI to the new one
  3. Detach the old VDI from the VM and attach the new one
  4. Expand the partition on the VDI
  5. Boot the VM and make sure everything works
  6. Once you’re sure the new VDI is working properly, delete the old one.

So, let’s break those steps down a bit…

To create a new VDI, you can either use the Virtual Media Manager in the GUI (look in the File menu), or you can use VBoxManage in a terminal window, like so (note: if you’re using a Windows host, open a command window and run: cd “C:\Program Files\Sun\xVM VirtualBox”):

VBoxManage createhd –filename my_filename.vdi –size 50000 –remember

This will create a 50GB (50,000MB) dynamic VDI called “my_filename.vdi” and store it in the default location (on Linux, this is usually ~/.VirtualBox/HardDisks). The ‘–remember’ flag will automatically register the VDI with VirtualBox, so that we can use it later.

Now that we’ve created the new VDI, we need to clone the old one to it. You have to use VBoxManage for this step, since cloning isn’t supported in the GUI.

VBoxManage clonehd old.vdi new.vdi –existing

Obviously you should replace “old.vdi” and “new.vdi” with the appropriate filenames. You can either specify absolute paths, or just specify the filenames if the VDIs are stored in the default folder. This process will take some time, so grab some coffee and we’ll pick back up in a few minutes…

Once you’ve cloned the VDI, you need to attach it the VM (and detach the old one). It’s probably easiest to do this in the settings page for the VM in the GUI, but it can also be done with VBoxManage. If your VDI is attached to the Primary Master IDE slot for the VM (which is the default), then you can use the following commands:

VBoxManage modifyvm MyVMName –hda none

VBoxManage modifyvm MyVMName –hda new.vdi

Obviously you’ll want to replace “MyVMName” and “new.vdi” with the appropriate values for your set environment. Once you’ve done this, you’ll need to expand the partition on the new VDI, to let it know it can use all the new space you’ve given it. The details of how to do this are outside the scope of this post, but the general idea would be to boot the VM with some sort of partitioning tool (GParted is a good Free option) and use that to expand the partition.

Once you’ve expanded the partition, boot the VM normally and make sure everything works. Once you’re sure everything works, unregister and delete the old VDI (again, this is easy enough to do in the GUI, but you can use VBoxManage as well).

VBoxManage closemedium disk old.vdi

rm /path/to/old.vdi

That’s all there is to it. The obvious downside to this method is that it takes up some extra disk space during the process, but if you’re looking to expand your VDI by any significant amount anyway, then this probably isn’t an issue.