Linked Clones, what are they

With the release of View 3 VMware have added a couple of killer features, today I will attempt to explain Linked Clones. first some basics.
What are Clones?
At its most basic a clone is a copy of an existing, or parent, virtual machine. When the cloning operation is complete, the clone becomes a separate virtual machine with unique identity of its own.

The concept of Linked clones was introduced with Workstation and then introduced into the enterprise with Lab Manager, this now means that are two types of clones; Traditional or Full Clones and Linked Clones.

What is a Full Clone

A full clone is an independent but identical copy of a virtual machine that shares nothing with the parent once the cloning operation is complete: the ongoing operation of a full clone is entirely separate from its parent. The fact that it is identical required a process like Sysprep to be run against it prior to use.

What is a linked clone?

Well Linked Clones are basically Snapshots on steroids.

For a slightly more technical look

A Linked Clone is a copy of a virtual machine that continues to share the virtual disks of its parents but basically runs of a snapshot. This special snapshot is often referred to as a Redo or Diff Disk. However because it is a Snapshot or difference disk the clone always requires access to the parent’s disk and therefore the term Linked.

As you can see the underlying technology is more akin to a snapshot than a clone but here is the magic you can have more than one active Snapshot coupled to the parent disk,

It can be seen that a major advantage of this is that each as linked clone operates as an independent virtual machine, a active guest can take up 100’s or MB of SAN storage rather than 10’s of Gigabytes utilised using traditional Cloning technology.

So how do we create this manna from heaven?

Well with the new VMware View Composer which is offered with the premier bundle of VMware View 3. View Composer offers the key benefits such as storage reduction, better OS management, and rapid deployment capabilities for virtualized desktops. This technology reduces duplicate storage of virtual machines’ data transparently to the virtual machine kind of like TPS for the Disk but not quite ;). To be exact, View Composer allows multiple VMs to share common data in a single base disk while maintaining separate storage for the data written by each virtual machine.

Parent Image and Replica

People familiar with the later workstations versions will already be aware of the concept of Linked clones and posibably understand the technology behind it, However, Linked clones created with View Composer utilise a slightly different process, this makes them much more scalable that WS linked clones. Under VMware Composer an extra layer is inserted between the parent and the linked clone. Linked clones are linked to a full clone virtual disk called a replica, these replicas are created in each datastore utilised for Desktop delivery.

There is a replica is created for each variant of the desktop image used in the deployment. Therefore, if both Windows XP and Vista, are deployed on a single LUN, there will be two replica disks created on that LUN, one for each OS.

Hand on a second, this is a replica, how are these machines made unique, VMware have even thought of that, there is a mini customisation run on each provisioned clone giving it a new computername, SID and joining it to the domain.

Image Management with View Composer

View Composer offers the following OS image management tasks:

  • Recompose image
  • Refresh image
  • Rebalancing

The Recompose function allows the desktop administrator to update the parent virtual machine and push the new version of the image out to all or a subset of users and desktops. Recomposing desktops can be used to achieve a variety tasks, such as:

  • Applying OS or software patches
  • Applying service packs
  • Adding additional software
  • Making virtual hardware changes
  • Upgrading OS versions

Refresh is the process of resetting linked clones back to the initial state of the parent virtual machine without adding additional software or patches or making other changes. Refreshing desktops can also be used to reduce the size of linked clones that have grown over time. Several options are available for initiating a desktop Refresh such as:

  • Refresh on demand
  • Refresh when a specific size has been reached
  • Refresh as a timed even

This function greatly simplifies the storage management by allowing administrator to relocate virtual machines from one LUN to another.  This triggers a move of the linked desktop(s) to another LUN and is akin to Storage vMotion, the move recreates the replica on the LUN.  Another benefit of rebalancing is that desktops are refreshed automatically

One important point to note is that rebalancing is not an online proces and therefore desktops need to be powered off.

Storage over-commit.

Storage Over-commit determines how aggressively virtual machines are allocated to available free space. The more aggressive the Over-commit, the more virtual machines are placed on a data store with free space.
There are four levels of over-commit these being:

  • None
  • Conservative (default)
  • Moderate
  • Aggressive

However, as more virtual machines are assigned to the available free space, management of the storage environment need to a actively addressed to ensure that the Datastores do not run out of space, In non-persistent use cases, where virtual machines are always refreshed and reset to their initial state, this is less of an issue than for more persistent virtual desktops.


All in all the inclusion of linked clone technology is a vast improvement for virtual desktop management. Storage needs are drastically reduced, thereby reducing the cost of a VDM implementation.
My only gripe is that it costs an extra $100 per seat. Every customer of VMware View needs this product it should have been bundled with the standard version

4 thoughts on “Linked Clones, what are they”

  1. unfortunately those who know the answer to you question Alan, Are not allowed to say as they are under NDA, That said as Lab Manager has a very similar concept as its mainstay, I personally do not think it likely they will be moving it in to ESX4, however that said, for Terminal Servers this could be “Manna from Heaven” :D. VMware take note.

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