Snapshots

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By the end of this session, learners will be able to:

  • Define VM snapshots and explain how they work internally
  • Create, revert, and delete snapshots safely
  • Interpret snapshot trees and understand delta disks
  • Use snapshots to recover from configuration failures
  • Apply snapshot best practices and avoid common pitfalls

1. What Is a Snapshot? (With Diagram)

A snapshot is a point-in-time record of a virtual machine’s state, mainly its disk contents and sometimes its memory.

Snapshot Concept Diagram

Before Snapshot
----------------
[ VM Disk.vmdk ]
       |
       v
 Running VM

After Snapshot
--------------
[ VM Disk.vmdk ]  (Read-only)
       |
       v
[ Delta Disk 0001.vmdk ]
       |
       v
 Running VM

Key idea:

  • The original disk is frozen
  • All new changes go into a delta disk
  • Reverting discards delta changes

2. Snapshot Tree Explained (Visual)

Snapshots can form a tree if multiple snapshots are taken.

Base Disk
   |
   +-- Snapshot A
   |       |
   |       +-- Snapshot B
   |
   +-- Snapshot C

Meaning:

  • Snapshot B depends on Snapshot A
  • Deleting Snapshot A affects Snapshot B
  • Longer chains = slower disk performance

3. Snapshot vs Backup (Comparison Table)

Feature Snapshot Backup
Independent No Yes
Long-term use No Yes
Fast rollback Yes No
Production-safe Limited Yes

Snapshots are temporary safety checkpoints, not archival storage.

4. When to Use Snapshots (Use Cases)

Good use cases:

  • Before software installation
  • Before configuration changes
  • OS updates
  • Training labs
  • Testing risky commands

Bad use cases:

  • Databases in production
  • Long-term storage
  • Disaster recovery

5. Advanced Snapshot Topics

5.1 Memory Snapshots

When taking a snapshot, you can choose to include the VM's RAM state.

  • Advantage: Reverting restores the VM exactly where it was (no reboot needed).
  • Disadvantage: Takes significantly more host disk space and takes longer to create.

5.2 Quiescing and Guest OS Interaction

In enterprise environments (VMware/Hyper-V), hypervisors use Guest Tools to "quiesce" the filesystem before taking a snapshot. This ensures that the filesystem is in a consistent state (buffers flushed) so that databases don't get corrupted.

5.3 Snapshot Consolidating

When you delete a snapshot, the hypervisor doesn't just delete files; it merges the delta changes back into the parent disk. This process is called Consolidation. If consolidation fails, the VM may experience "stun" (brief freezes) or disk errors.

6. Practical Lab: Snapshot Recovery with a Web Server

Lab Objective

Use a snapshot to recover a Linux VM after breaking a web server configuration.

Lab Environment

  • Linux VM (Ubuntu 20.04+ recommended)
  • User with sudo privileges
  • Internet access

Part 1: Install a Web Server (Apache)

Step 1: Update Packages

sudo apt update

Step 2: Install Apache

sudo apt install apache2 -y

Step 3: Start and Enable Apache

sudo systemctl start apache2
sudo systemctl enable apache2

Part 2: Take a Snapshot

Snapshot Action (Hypervisor GUI)

  • Snapshot name: Working Web Server
  • Description: Apache installed and running correctly

Part 3: Break the Web Server Configuration

Step 1: Edit Apache Configuration

sudo nano /etc/apache2/ports.conf

Step 2: Introduce an Error

Change Listen 80 to Listen eighty.

Step 3: Restart Apache

sudo systemctl restart apache2

Expected result: Service fails to start.

Part 4: Revert to Snapshot

  1. Power off VM.
  2. Revert to Working Web Server.
  3. Power on VM.
  4. Verify Apache is working again (sudo systemctl status apache2).

7. Advanced Lab: Analyzing Delta Disks

  1. Take a snapshot of your VM.
  2. Find the VM's storage folder on your Host OS.
  3. Identify the new file created (often ending in .vmdk or .vdi but with a long hex string in the name).
  4. Note its size.
  5. Create a large file inside the VM (dd if=/dev/zero of=testfile bs=1M count=100).
  6. Check the delta file size on the host again. Notice how it grew while the base disk remained unchanged.

8. Best Practices Checklist

✔ Take snapshots before risky changes ✔ Use clear names and descriptions ✔ Delete snapshots when done ✘ Do not keep snapshots long-term (more than 24-72 hours) ✘ Do not use snapshots as backups

Session Wrap-Up

Snapshots give virtual machines a rewind button. In this session, we explored the mechanics of delta disks, the risks of long-term snapshots, and performed a recovery lab.

Today’s key idea:

Snapshots protect moments. Clones and templates create futures.

Network Modes Cloning