Cloning

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Session Goal

By the end of this session, students will be able to clone virtual machines correctly, choose between full and linked clones based on use case, and prepare a reusable “golden image” template without causing identity conflicts on the network.

Think of a VM not just as a disk and some RAM, but as a fully formed digital identity. Cloning copies that identity unless you deliberately erase the fingerprints first 🧬.

1. Why Clone Virtual Machines?

Cloning is used whenever you need multiple identical systems without repeating installation and configuration work.

Common scenarios include:

  • Building identical lab machines for students
  • Creating test environments that mirror production
  • Rapidly deploying servers or desktops
  • Preserving a clean baseline before experiments

Instead of reinstalling an OS ten times, you build once, perfect it, then copy it.

2. Types of VM Clones

Virtualization platforms support two main cloning strategies. The difference lies in how disk data is stored and shared.

2.1 Full Clones

A full clone is a completely independent copy of a virtual machine.

How it works

  • The entire virtual disk is duplicated
  • The clone has no dependency on the original VM

Advantages

  • Safe and isolated
  • Parent VM can be deleted with no impact
  • Best choice for long-term or production use

Disadvantages

  • Consumes significant disk space
  • Takes longer to create

Example A 20 GB VM cloned ten times consumes roughly 200 GB.

2.2 Linked Clones

A linked clone shares the original VM’s base disk and stores only changes in a small differencing disk.

How it works

  • Parent VM disk becomes read-only
  • Each clone writes changes to its own delta file

Advantages

  • Extremely fast to create
  • Minimal initial storage usage
  • Ideal for labs, classrooms, and testing

Disadvantages

  • Dependent on the parent VM
  • If the parent is deleted or corrupted, all linked clones fail
  • Not suitable for long-term independence

Example One 20 GB parent + ten linked clones may initially use only ~21 GB total.

Comparison Table: Full Clone vs. Linked Clone

Feature Full Clone Linked Clone
Disk Type Full Copy Differencing Disk (Delta)
Parent VM Independent Dependent
Storage High (full disk size) Low (only changes stored)
Creation Slower Faster
Use Case Production, Archiving Testing, Labs, VDI

Visualizing Clone Types

[Parent VM Disk]
       |
       |--- [Full Clone VM Disk 1] (Independent Copy)
       |
       |--- [Full Clone VM Disk 2] (Independent Copy)

[Parent VM Disk] <--- [Linked Clone VM Delta Disk 1] (Reads from Parent)
       |
       |--- [Linked Clone VM Delta Disk 2] (Reads from Parent)

3. Golden Master and Templates

A golden master VM is a carefully prepared system that serves as the source for all clones.

Standard Workflow

  1. Install OS
  2. Apply updates
  3. Install required software
  4. Configure system defaults
  5. Generalize the system
  6. Shut down
  7. Clone or convert to a template

The generalization step is critical. Without it, all clones will share the same internal identity.

4. Generalization and Identity Conflicts

Cloning without generalization causes problems such as:

  • Duplicate hostnames
  • Conflicting IP leases
  • Authentication and security issues
  • Broken domain joins

This happens because operating systems store unique identifiers that must be reset.

5. Generalization by Operating System

5.1 Windows: Sysprep

Windows stores identity data in the Security Identifier (SID) and other system-specific values.

Sysprep removes this identity so Windows can regenerate it on next boot.

Typical command

sysprep /generalize /oobe /shutdown

What this does

  • Removes SID
  • Resets hardware-specific data
  • Starts Windows setup on next boot

This is the only Microsoft-supported method for cloning Windows systems.

5.2 Linux: machine-id and Cloud-Init

Most modern Linux distributions use systemd, which stores a unique identifier in:

/etc/machine-id

Manual Generalization (Lab-Scale)

sudo truncate -s 0 /etc/machine-id

On next boot, the system generates a new ID automatically.

Cloud-Init (Enterprise / Cloud)

Cloud-init is the industry standard for large-scale Linux deployments.

It can:

  • Generate unique machine IDs
  • Set hostnames
  • Inject SSH keys
  • Run startup scripts
  • Install packages on first boot

This is how AWS, Azure, and OpenStack deploy thousands of VMs without collisions.

6. Linked Clones in Education and VDI

Linked clones shine in environments where:

  • VMs are temporary
  • Storage efficiency matters
  • Rapid provisioning is required

Virtual Desktop Infrastructure (VDI) uses this technique to deploy thousands of desktops from a single golden image, each appearing unique to the user while sharing a common base.

7. Lab Exercise

Part 1: Prepare the Master VM (Linux)

  1. Boot the master VM
  2. Run:

sudo truncate -s 0 /etc/machine-id

  1. Shut down the VM

Part 2: Create Clones

  1. Create one Full Clone

  2. Record disk usage on the host

  3. Create one Linked Clone

  4. Record disk usage on the host

Part 3: Verification

  1. Boot all three VMs
  2. Confirm:
  3. Unique hostnames

  4. Unique IP addresses

  5. On the parent VM:

touch /tmp/parent-test

  1. Verify the file does not appear in either clone

This confirms logical disk isolation even when a base disk is shared.

8. Corrected and Stable References

These references are authoritative and unlikely to break or mislead students:

Snapshots Automation