Tech Talk

What is a linked clone?

In this 2 part article, we will be discussing linked clones, which are an integral part of VMware View. Part 1 will contain an overview of what a linked clone is. Part 2 will cover the operations that can be performed on a linked clone and how to maintain them.

So, what is a linked clone?

It’s the weekend and you’re out at a dinner party with several new friends. The conversation goes around the table from person to person and everyone talks about who they are and what they do for a living. The conversation gets around to you and you say, “I’m a VMware View Administrator”. If your friends are like mine, you will get blank, questioning stares in return. As a VMware View Administrator, what do you tell people what you do when they ask and how do you explain it?

I tell them I’m a tree tender.

To most people, the concept of virtualization is foreign, but a simple explanation is normally enough to understand what a virtual machine is and provide a basic idea of how it works. But what about a linked clone? This is where we use our tree analogy.

Simplifying things, if you were to describe a tree to someone, how would you do it?

A tree has a central core or trunk, several leaf-covered branches coming off at the top, and at the base in the ground it has roots. A linked clone can be thought of in a similar way.

The Tree Trunk

This can be known as the template, golden image, or base image. It is a virtual machine (VM) that is installed with all of the appropriate Windows updates, applications and patches that your users require. This is setup and configured to be a generic VM from which our linked clones will branch.

The Branches

This is the linked clone replica. This is a duplicate copy of the snapshot taken of the base image.  The replica is used as the “base” for all linked clones attached to it, for the terms of space saving and efficiency. Each linked clone pool will have at least one replica, one per snapshot in use.

The Leaves

Each linked clone VM within a pool is a leaf. Each leaf is attached to a branch which is attached to a trunk. The linked clone VM is not a standalone VM, but is a combination of the replica and a difference disk. The difference disk contains every disk write made after the replica is created.

The Roots

This is your vCenter. View Connection Server provides access, View Composer creates requests for cloning and customization, but your vCenter runs the whole show and makes things happen. Everything hooks into your vCenter.

How does it all work?

First, you need to create the template and prepare it for use. This includes installing VMware tools and the View Agent. Once you have all Operating System and application installed and updates complete, the template is ready, to be shut down and it’s snapshot is taken.

Within the View administrator, you will configure the pool to point at the specific base image and select the appropriate snapshot. Multiple pools can point to different images, the same image and snapshot, or the same image and different snapshots.

Once the pool is created, Composer will then request that vCenter clone a replica machine for use by the pool, and create the number of VMs needed to satisfy its pool requirements. This is where the linked clone machine is different than a standard VM. The replica contains everything that is needed to run Windows. It has all of the OS files, boot files and applications. The linked clone VM attaches to this so every linked clone shares the same Windows installation. A disk is created that contains the changes made for an individual VM. This checkpoint contains the domain and naming information and other basic information (the customization information) required to differentiate it from other linked clones attached to the same replica.

The linked clone can be customized in one of 2 ways. Sysprep (a Microsoft tool) uses a vCenter customization template that can be utilized to provide information on customizations to be performed. The second option is to use Quickprep which is Composer’s built in customization tool, and is generally recommended unless a Sysprep-specific customization is required.

At this point, the difference disk is created and all disk writes from that point on are saved to the difference disk. The replica is not modified, and the checkpoint contains all of the customization information that would be needed in the event the linked clone needs to be refreshed.

Watch this space for part 2 coming in about a week’s time. Thanks for reading!

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