Snapshots
Point-in-time captures of a VM, what they include, and what they are not.
A snapshot captures a VM at a moment so you can return to it. Take one before an upgrade, a configuration change, or anything you might want to undo.
Disk-only or with memory
| Kind | Captures | Reverting gives you |
|---|---|---|
| Disk only | The disks | The VM as if it had been powered off at that moment |
| Disk and memory | Disks plus RAM | The VM running, exactly as it was |
Disk-only is the default and the right choice most of the time. A memory snapshot is larger by the size of the guest’s RAM and takes longer, but it restores a running machine — no boot, no service restart, no lost in-memory state.
Take a memory snapshot when the thing you are protecting is in RAM. Take a disk snapshot when the thing you are protecting is on disk, which is nearly always.
Snapshots form a tree
Each snapshot records its parent. Taking a snapshot, reverting, then taking another gives you a branch rather than a line. The VM’s snapshot list shows the relationships.
This matters when deleting: removing a snapshot with children is not the same as removing a leaf.
Creating one
Open the VM, choose Snapshots → Create, give it a name and a description, and decide whether to include memory.
Name them for why, not when — the timestamp is already recorded. “before 14.2 upgrade” is useful in three weeks; “snapshot3” is not.
Reverting
Reverting discards everything written since the snapshot was taken.
Warning — There is no undo. A revert throws away all changes after that point, including data written by applications that believe it is safely committed. Shut the guest down cleanly first if you can, and be certain you have the right snapshot.
Deleting
Snapshots consume space that grows as the VM diverges from them. Delete them when the change they were protecting has proved itself.
Quiescing
A snapshot of a running VM captures the disk as it is at that instant — which may include a database mid-write. With guest tools installed, the platform can ask the guest to flush and freeze its filesystems first, producing a consistent image.
Without guest tools, a disk snapshot of a running VM is crash-consistent: equivalent to pulling the power. Most filesystems and databases recover from that, but recovery is not the same as consistency. See Templates and images for installing guest tools.
What snapshots are not
Warning — Snapshots are not backups. They live on the same storage as the VM. A failure that takes out the storage takes the snapshots with it. They protect against change, not against loss.
For protection against loss, use replicated storage plus one of:
- Disaster recovery — replication to a second site.
- Backup and restore — for the control plane’s own database.
Nor are snapshots a substitute for capacity planning: a VM left with months of snapshots consumes storage proportional to how much it has changed.
Automatic snapshots
A snapshot policy takes them on a schedule and prunes old ones, so the discipline does not depend on anyone remembering. A policy has a schedule, a retention count — how many to keep, default 5 — and whether snapshots are disk-only, which they are by default.
See Snapshot policies.