Snapshots let you capture a VM’s state at a specific moment so you can roll back if something goes wrong (updates, risky changes, tests). In KVM with libvirt there are two main families:

  • Internal snapshots: the snapshot content is stored inside the qcow2 file itself. They usually require the VM to be powered off and only work with qcow2 storage (not raw, LVM, etc.).
  • External snapshots: they create separate overlay files (qcow2), these are the most used for hot snapshots (running VM). They allow more flexible flows and can be consolidated later (blockcommit).

Additionally, a snapshot can be:

  • Disk-only (most common): saves the state of the disk(s).
  • With memory: includes RAM content and CPU state to resume exactly as it was (heavier and slower).

General recommendations:

  • For hot snapshots, use external snapshots with --disk-only and, if possible, --quiesce with qemu-guest-agent.
  • Limit the overlay chain depth, consolidate early with blockcommit to avoid performance degradation.
  • Do not use snapshots as a replacement for full backups.

Requirements and quick checks

# List the disks attached to the VM
virsh domblklist debian13

# Check the disk format (important for internal snapshots)
qemu-img info /var/lib/libvirt/images/debian13.qcow2

# Check if the guest agent is working (for --quiesce)
virsh domfsinfo debian13            # requires qemu-guest-agent
virsh qemu-agent-command debian13 '{"execute":"guest-info"}' --timeout 5

Notes:

  • Internal snapshots require qcow2 and typically a powered-off VM.
  • --quiesce requires qemu-guest-agent installed and configured in the guest.

This is the most common flow before a production update.

# Hot disk snapshot with libvirt metadata
virsh snapshot-create-as \
	--domain debian13 \
	--name snap-pre-upgrade \
	--description "Before apt full-upgrade" \
	--disk-only \
	--atomic \
	--quiesce \
	--live

# If you don't have qemu-guest-agent, omit --quiesce
# virsh snapshot-create-as --domain debian13 --name snap-pre-upgrade \
#   --description "Before apt full-upgrade" --disk-only --atomic --live

Verify the overlays created and the snapshot metadata:

virsh snapshot-list debian13
virsh domblklist debian13

Typical domblklist output after an external snapshot:

 Target   Source
----------------------------------------------
 vda      /var/lib/libvirt/images/debian13-snap-pre-upgrade.qcow2

That indicates that writes now go to the overlay (the base file remains protected).

Create an internal snapshot (powered-off VM, qcow2)

Useful for simple scenarios with qcow2 storage and no need for hot snapshots.

# Power off the VM
virsh shutdown debian13 && virsh domstate debian13

# Create an internal snapshot explicitly for disk vda
virsh snapshot-create-as \
	--domain debian13 \
	--name snap-interno \
	--description "Before changing fstab" \
	--diskspec vda,snapshot=internal

# Start again if appropriate
virsh start debian13

For internal snapshots, you can also use qemu-img snapshot with the VM powered off, but virsh is recommended to keep metadata consistent in libvirt.

List, show info, and current snapshot

# List the VM's snapshots
virsh snapshot-list debian13

# Show information about a specific snapshot
virsh snapshot-info debian13 --snapshotname snap-pre-upgrade

# Current snapshot (to which the present state points)
virsh snapshot-current debian13

Revert to a snapshot

With snapshots that include memory you can resume the exact state, with disk-only snapshots it’s safer to revert with the VM powered off.

# Conservative option for disk-only snapshots: power off, revert, and start
virsh shutdown debian13
virsh snapshot-revert debian13 snap-pre-upgrade
virsh start debian13

# If the snapshot includes memory, you can indicate the state after reverting
# virsh snapshot-revert debian13 snap-con-memoria --running   # resume
# virsh snapshot-revert debian13 snap-con-memoria --paused    # paused

Tip: after reverting, validate applications and filesystems.

Delete snapshots

# Delete a specific snapshot (metadata in libvirt)
virsh snapshot-delete debian13 --snapshotname snap-pre-upgrade

# Delete a snapshot and its descendants
# virsh snapshot-delete debian13 --snapshotname snap-pre-upgrade --children

Important: For external snapshots, deleting the metadata does NOT automatically consolidate overlays. To return to a simple chain, use blockcommit.

Clean up overlays (consolidate) with blockcommit

After verifying everything is fine, it’s advisable to consolidate the overlay into the base disk to avoid long snapshot chains that penalize performance.

# Identify the target (e.g., vda)
virsh domblklist debian13

# Start blockcommit live and pivot when it finishes
virsh blockcommit debian13 vda --active --verbose --pivot

# Check job status (optional)
virsh blockjob debian13 vda --info

After --pivot, the VM writes directly to the base file again and the overlay can be deleted if it’s no longer referenced.

Quick demo (flow idea)

# 1) Create external snapshot before making changes
virsh snapshot-create-as --domain debian13 --name snap-demo --description "Before tests" --disk-only --atomic --live

# 2) (Inside the VM) apply changes
# sudo apt update && sudo apt -y full-upgrade

# 3) If something fails, revert
virsh shutdown debian13
virsh snapshot-revert debian13 snap-demo
virsh start debian13

# 4) If everything went fine and you don't need the snapshot, consolidate and clean up
virsh blockcommit debian13 vda --active --verbose --pivot
virsh snapshot-delete debian13 --snapshotname snap-demo

Quiesce with qemu-guest-agent (better snapshots)

To reduce the risk of inconsistencies (especially in databases), use --quiesce with the guest agent.

Inside the guest:

# Debian/Ubuntu
sudo apt install -y qemu-guest-agent

# RHEL/CentOS/Rocky/Fedora
sudo dnf install -y qemu-guest-agent

# Ensure the service is active
sudo systemctl enable --now qemu-guest-agent

In the VM definition (it usually comes in modern templates), the agent’s virtio channel must exist. Verify:

virsh domfsinfo debian13    # if it responds, the agent is working

Then create snapshots with --quiesce.

Select disks and exclusions

You can limit the snapshot to a specific disk or exclude one:

# External snapshot of disk vdb only (useful for data)
virsh snapshot-create-as \
	--domain debian13 \
	--name snap-datos \
	--description "Data only" \
	--disk-only --atomic --live \
	--diskspec vdb,snapshot=external

# Exclude an ephemeral disk (e.g., swap)
# --diskspec vdc,snapshot=no

Common problems and how to solve them

  • --quiesce fails: install and enable qemu-guest-agent inside the guest and check virsh domfsinfo.
  • Unsupported storage: internal snapshots require qcow2, for raw/LVM/ceph use external snapshots.
  • Snapshots with memory are heavy: they can take time and consume a lot, assess whether you really need memory.
  • Deep overlay chain: I/O impact, consolidate with blockcommit early.
  • Reverting a disk-only snapshot hot: may corrupt data, power off before reverting.
  • Not a replacement for backups: snapshots do not protect against host failures or long-standing silent corruption.

References