Time and again, when we ask “Do you have a backup of this server?”, we hear: “Yes, we take a snapshot every week.” Snapshots are a very useful tool, but they are not backups. On the day the host’s disks fail or ransomware reaches the storage, the snapshots disappear along with the VM, and only then does the difference become clear.
What exactly is a snapshot?
A snapshot captures the state of a virtual machine’s disk (and, if selected, its memory contents) at a single point in time. From that moment on, new changes are written to a separate file or layer, and the original disk stays untouched. Whenever needed, you can revert the VM to that moment. In Hyper-V this feature is called a Checkpoint, and in Proxmox its availability depends on the storage type.
Here is the key point: a snapshot is not an independent copy. It only stores the differences, depends on the original disk to work, and usually sits on the same storage.
Why a snapshot can’t replace a backup
- Same location, same fate: The snapshot sits next to the original disk. A RAID failure, lost storage or an accidentally deleted VM destroys both at once.
- Dependence on the original disk: If the base disk file is damaged, the snapshot is useless on its own.
- Performance degrades over time: The older a snapshot gets and the longer its chain, the larger the change file grows and the slower disk operations become. Consolidating or merging a very large snapshot can also take a long time and require extra space.
- Filling up storage: A forgotten snapshot can slowly fill the storage and bring every VM on it to a halt.
- No long-term retention: A proper backup keeps several copies from different days and weeks in another location; snapshots are not designed for that.
The right use of snapshots
Snapshots are excellent for short-term, reversible changes:
- Before installing an OS update, upgrading software or making a major configuration change, take a snapshot.
- Make the change and verify that the service works correctly.
- If something goes wrong, revert to the snapshot; if everything is fine, delete the snapshot as soon as possible so the changes merge into the original disk.
A simple rule: keep a snapshot for a few hours or a few days at most, not weeks or months. Check your platform vendor’s exact recommendation in the documentation for your version.
Be careful reverting snapshots on critical servers
Reverting to a snapshot sends the VM back in time. On a database server, every transaction after that moment is lost. On a domain controller, restoring an old copy can disrupt replication with the other controllers; newer versions of Windows Server have safeguards on supported platforms, but it is best not to do this without a solid understanding. Also, a regular snapshot is not necessarily coordinated with the applications inside the VM; if it is captured in the middle of a database write, the result may not be fully consistent.
What does a real backup look like?
Interestingly, backup software such as Veeam or Proxmox Backup Server also uses snapshots behind the scenes, but only for a few minutes, to read the data in a consistent state and move it to a separate repository. After that, the snapshot is deleted. A real backup:
- is stored on storage separate from the main host,
- can be restored independently, without needing the original disk,
- has multiple copies from different dates,
- has at least one off-site or immutable copy,
- is taken in coordination with the OS and applications (application-aware) so databases stay intact.
Checklist
- You have reviewed the existing snapshots on all hosts and deleted the old ones.
- For each snapshot, you have noted the reason and its deletion date in its description.
- No server relies only on snapshots; each has a separate backup.
- Backups are on storage separate from the virtualization host.
- You monitor free space on your storage.
A snapshot is the seat belt during changes, and a backup is the insurance policy for all your data; you need both, but neither replaces the other. ITPlus724 engineers can review the snapshots and backups of your virtual machines and implement the right backup plan.