A snapshot before a big Ubuntu upgrade is one of virtualization's genuine superpowers: if the upgrade goes wrong, you rewind the whole machine. But the same feature, left running for months, quietly eats your datastore and slows the VM it was protecting. This page teaches the ritual - and the discipline that has to come with it.
Here is the upgrade scene every administrator knows: the update runs, something deep breaks, and the evening disappears. An ESXi snapshot taken five minutes earlier turns that story into a shrug - revert, and the machine is exactly as it was. The catch is that a snapshot is a living thing: from the moment you take it, the VM's changes pile up in a side file that grows until you deal with it. Snapshot literacy is two skills - taking them at the right moment, and closing them on purpose.
A snapshot freezes the VM's disk state at a moment. From then on, the VM writes its changes into a new delta file, and the original disk sits untouched underneath. Revert throws the delta away and you are back at that moment; delete (consolidate) folds the delta into the base disk and you keep the present.
Two consequences flow from that mechanism. First, while a snapshot lives, the VM's storage work splits across files - long snapshot chains cost real performance. Second, the delta grows with every change the VM makes, at the VM's own pace: a busy server can grow its snapshot by gigabytes a day. The snapshot is a tab being run up against your datastore, and the bill arrives when you consolidate - or when the datastore fills first.
sudo apt update and sudo apt upgrade for routine updates, or the release-upgrade path for a version jumpOnce the upgraded VM has proved itself - a few days of normal work is a fair trial - close the snapshot deliberately:
If you find yourself afraid to delete a snapshot "just in case", hear what that fear is telling you: the VM needs a real backup. Which is the next section.
The snapshot lives on the same datastore, in the same host, as the VM it protects. A dead datastore, a dead host or a ransomware event that reaches the hypervisor takes the VM and its snapshots together. A backup is a copy somewhere else.
So the standing pair of rules:
Keeping a snapshot "as the backup" for six months delivers the worst of both: no disaster protection, and a datastore slowly filling with delta files.
If you inherit a host, audit snapshots early. The symptoms of snapshot debt: a datastore shrinking although the VMs look small (their delta files are not small), a VM that feels slower each month (its disk chain is long), and consolidation jobs that run for hours because months of changes must be merged.
The recovery is patience, not heroics: pick a quiet window, delete the oldest snapshots one at a time, let each consolidation finish, and watch free space return. Then set the household rule that prevents relapse - every snapshot gets a name, a reason and a planned deletion date, and the calendar is honoured.
The full process on a live ESXi host - creating the virtual machine, installing Ubuntu and fixing the common problems, exactly as they appear on screen.
Watch Full VideoThink in days, not months - long enough for the change it protects to prove itself, commonly a few days to about two weeks for an upgrade. The snapshot's delta file grows with every write the VM makes the whole time it exists, costing datastore space and disk performance. Once you trust the upgraded system, delete the snapshot and let it consolidate.
No. Snapshots live on the same datastore as the VM itself, so the same failure takes both. A snapshot protects against a bad change - a failed upgrade, a broken setting. Disaster protection needs a copy on separate storage: a backup job or an exported copy of the VM, as the OVF move guide in this series describes.
Deleting keeps the present: the changes recorded since the snapshot are merged into the VM's main disk and the snapshot disappears. Reverting discards the present: the VM returns to exactly the state captured in the snapshot, and everything written since - files, settings, data - is gone. Delete is the happy ending; revert is the ejector seat.