FULL VS LINKED
Independent or dependent
A full clone copies all the disk images and runs without the source. A linked clone uses new differencing disks based on the source's disks.
Pick a methodWHAT THE DOCS SAY
Copy the machine,
not its identity.
Cloning a VM takes minutes. Cloning it well takes a little thought.
FULL / LINKED two kinds of clone
72hours Longest time to keep a single snapshot
- Full clones stand alone
- New MACs, new IDs
- Snapshots are not backups
- Safer copies of virtual machines
A clone can be a fully independent copy or a lightweight copy that leans on the original's disks. Either way, the copy needs its own network addresses and system IDs before it shares a network with its parent.
THE SHORT ANSWER
Two kinds of clone,
one golden rule.
A clone copies everything, including identifiers.
Change them before both machines go online.
IDENTITY
New MAC addresses
VirtualBox's default is to generate new MAC addresses for every network adapter, the best choice when the source and clone share a network.
System IDs
SNAPSHOTS
Not a backup
VMware's guidance says a snapshot is only a change log of the original disk. Delete the base disk and the snapshot alone cannot restore the VM.
See the limitsA QUICK METHOD CHECK
Which copy fits
the job?
Pick what you want to do to see the approach the vendor documentation points to. The bar shows how independent the result is from the original VM.
Based on the VirtualBox user manual, VMware snapshot guidance, Microsoft's Sysprep documentation and the machine-id manual page.
FOUR HABITS FOR CLEAN CLONES
A tidy clone
in four steps.
These come from the documentation for common hypervisors and operating systems. Product menus differ, but the principles carry across.
Shut the source down
VirtualBox disables its Clone menu while a machine is running.
Choose full or linked
Full for an independent copy; linked to save space when the source will stay put.
Give it new network addresses
Generate new MAC addresses so source and clone can share a network.
Reset system identity
Generalise Windows with Sysprep; on Linux images, leave /etc/machine-id empty so a new ID is made at boot.
TWO NUMBERS WORTH KNOWING
Snapshots have
a shelf life.
Two limits from VMware's own best practices,
and why they exist.
72hours
Longest time to keep a single snapshot
The snapshot file keeps growing the longer it is kept, which can fill the storage location and hurt performance.
AGE · VMWARE KB
2–3
Snapshots to use in a chain for good performance
A maximum of 32 snapshots is supported in a chain, but VMware recommends using only 2 to 3 for better performance.
CHAIN · VMWARE KB
Microsoft adds a limit of its own for images: Sysprep can be run up to 1,001 times on a single Windows 8.1 or Windows Server 2012 or later image, after which the image must be recreated.
WHY IDENTITY MATTERS
Two machines,
one name tag.
Operating systems keep unique IDs. Microsoft says generalising a Windows image removes computer-specific information such as installed drivers and the security identifier (SID), and that this is needed even when the target hardware is similar.
On Linux, the machine-id manual says each machine's ID should be unique. For images used on many machines, /etc/machine-id should be missing or empty in the image, so a fresh ID is generated at first boot.
Questions
Questions people ask
What is the difference between a full clone and a linked clone?
In VirtualBox, a full clone copies all dependent disk images and can run without the source VM; a linked clone creates differencing disks based on the source VM's disks.
Can I use a snapshot as a backup?
No. VMware's best-practice guidance says snapshots are only a change log of the original disk and are not sufficient to restore a VM if the base disks are lost.
Why does a cloned VM need a new MAC address?
VirtualBox generates new MAC addresses by default and says this is the best option when the source and clone must operate on the same network.
Do I need Sysprep before cloning Windows?
Microsoft says you must generalise a Windows image before deploying it to other PCs, to remove unique information such as the computer SID, even on similar hardware.
Can I clone a running VM?
In VirtualBox, the Clone menu item is disabled while a machine is running, so shut it down first.
Where the facts come from
Updated