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Using Immutable Image with a Differencing Disk

An immutable image, as its name implies, does not permanently record changes: changes are available only while the machine is running. As soon as it’s power cycled, changes are lost and the image its reverted to its original state. As in the case of snapshots, Virtual Box will use a differencing image to record changes to an immutable image. The differencing image is then reset to its original state at guest power cycle. The auto-reset feature of a differencing disk can also be turned off. Immutable images can be attached to as many running hosts as you wish: each guest they’ll be attached to will create a new differencing image to store their changes.

Using immutable images with differencing disks is one of the quickest way to deploy multiple instances of the “same” virtual machine. Since more than one virtual machine will be sharing the same underlying image you’ll also spare a good amount of storage space and use it as a way to reduce software installation and licensing for multiple users.

I, for example, had to deploy more than ten instances of a proprietary database: all of the instances could share the same immutable image since all of the instance would use the very same operating system.

To use an immutable image with differerencing disks, you have to go through several steps:

Creating A Virtual Machine

The first step to build your own immutable image is creating a virtual machine. This step is the same as usual:

Marking the Image as Immutable

Once you’re finished with your virtual machine you’ve got to mark the image as immutable. You need to release the image on Virtual Media Manage and then change the image type to immutable.

Attaching the Immutable Image to a Virtual Machine

Now you can attach your immutable image to a virtual machine. Every time you attach an immutable image to a virtual machine, Virtual Box will create a differencing disk for you that will store the changes recorded for a specific virtual machine.

As explained in the previous sections, the default differencing disks created by Virtual Box will be reset every time a virtual machine is stopped and restarted. Depending on your use case, you might want changes to be permanent.

Setting an Image Auto Reset Flag to Off

Every differencing disk has got an autoreset flag which, by default, is set to on. To set it to off you have to use the VBoxManage command:

$ VBoxManage modifyhd <uuid>|<filename> \

–autoreset off

Your differencing disk won’t “forget” any more the changes it records.

If you are deploying this system for a classroom of students and prefer all virtual machines restoring to a standard configuration after reboot, you can skip this step.

Conclusion: Using immutable images with differencing disks can be very helpful in many scenarios. Either if you’re deploying the system for a classroom of students, or if you’re simply installing virtual machines for your business staff, immutable images and differencing disk will help you spare a lot of storage space and, above all, a lot of your precious time.

For detailed example guide, view SUNDE Virtual Differencing Hard Disk Settings.

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