A recent post by the writer of The Grey Blog covers the topic VirtualBox: Using Differencing Disks and Immutable Images.

The blog begins with an explanation of the three-part image write mode, which is a property of every virtual disk image being used in guest storage. The three modes are normal; write-through; and immutable, each of which the blog explains in brief, including the advantages and disadvantages of each.

The immutable image finds the greatest favor with the blogger, given its "uncompromisable" character, which returns the images to their initial state, even during a power cycle. The blog claims that the immutable image can be used by multiple hosts and that they " ... * are the quickest way to deploy multiple (initially) identical guest instances."

The versatility and durability of the immutable image make it the choice of the blogger come time to discuss differencing disks. Here, the post reads, "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."

The blog outlines the steps involved in building an immutable image, which include:

• Creating a virtual machine
• Cloning the disk
• Marking the image as immutable
• Attaching the immutable image to a VM

Code examples and screen shots are provided to illustrate the process.

The post concludes with the comment that whether one is deploying many images or simply installing a couple of VMs, " ... immutable images and differencing disks will help you spare a lot of storage space ...."

More Information

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