Linked Clones in VMware

A powerful technology has been available from VMware for quite some time and yet it is not fully understood by many technical folks.  A virtual machine clone, like the word suggests, is a copy of an existing virtual machine (VM).  A cloned VM can take two forms: a linked clone or a full clone.  Since clones can be created at any moment in time, they provide many unique opportunities for managing the virtual environment while accomplishing tasks in a more productive manner.  This functionality can also be challenging if not properly managed. 

A full clone is a copy of a VM that is totally independent of the original, thus providing quick and easy provisioning and better performance than a linked clone.  A linked clone is a copy of a parent VM that shares the virtual disks of the parent VM in an ongoing manner and only maintains the differences in its own virtual disks.  Because of this, a linked clone requires the parent VM to function.  This conserves disk space and allows multiple virtual machines to use the same software installation within the parent.  Since a linked clone is made from a snapshot of the parent VM, ongoing changes to the virtual disk of the parent do not affect the linked clone and vice versa (the two VMs are effectively different branches on a tree).

Two inherent attributes of a VMware virtual machine are: a MAC address and UUID.  Even though clones are copies of other VMs, these values will be unique for every VM including both full and linked clones. 

Each clone type has its place in a virtual environment.  A full clone is generally used when a complete system needs to be distributed across multiple individuals such as training lab.  A linked clone, on the other hand, can provide collaboration through ease of transportability.  A baseline development environment can be created as the parent VM and distributed to the team members.  Linked clones can then be created for each specific development environment that may be needed without affecting the base VM.  In this manner, the developers can write code or even test code in their environment while not affecting the baseline.  The ability to distribute the smaller sized linked clones between resources also becomes a factor when one individual writes the code while another developer tests it.  The tester would receive the linked clone from the author and attach it to his copy of the common parent VM.  Another aspect of the linked clone technology is the potential of creating a linked clone from a linked clone thus providing a more dynamic environment.  Such technology is useful when an Operating System is setup as the parent VM with an application setup on a linked clone.  If testing is required for a specific service pack, another linked clone may be created from the application linked clone in order to perform the specified tasks.  While this ability is powerful, VMware warns that performance degradation can be expected when chaining together linked clones.

Clearly, if not managed properly, linked clones and linked clones of linked clones can get out of control.  By using good methodology and using the internal notes to document the clone information, your environment can be efficient and maintainable.  If you have questions about these methodologies (and how Razorleaf utilizes these features to streamline our test and development environments), or about other virtualization subjects, please contact us.  If you’ve used these techniques (or other interesting virtualization techniques), please leave us a comment.

Tags: test environment, virtual disk, Virtual Machine, virtual machine full clone, virtual machine linked clone, virtualization, VM, VMWare