When creating a snapshot in vSphere ESXi, you have the option to include the virtual machine’s memory. This is an important consideration as it impacts the snapshot’s creation speed, revert speed, and consistency.
Here’s a detailed breakdown of capturing memory in snapshots versus not:
Capturing Memory (Snapshot with Memory):
- Pros:
- Revert to an exact running state: Reverting to this snapshot restores the VM precisely to its state when the snapshot was taken, including all running applications and open files. Useful for testing or troubleshooting scenarios that require an exact replica.
- Application consistency: For certain applications, particularly databases, capturing memory helps ensure application-level consistency upon revert.
- Quicker recovery in some scenarios: Since the machine is already running, a revert doesn’t require a full boot.
- Cons:
- Slower snapshot creation: Capturing memory takes additional time, especially for VMs with large amounts of RAM.
- Larger snapshot files: Snapshots that include memory are significantly larger than those that don’t.
- May cause a brief VM unresponsiveness: The VM may experience a short pause (usually less than a second) while the memory state is captured. Latency-sensitive applications might be affected in some cases.
Not Capturing Memory (Snapshot without Memory):
- Pros:
- Faster snapshot creation: Snapshots are created much faster without capturing memory.
- Smaller snapshot files: Snapshot file sizes are smaller.
- Minimal impact on VM: There’s minimal performance impact on the VM during snapshot creation.
- Cons:
- Revert to a powered-off state: Reverting to this snapshot will start the VM from a powered-off state, similar to a cold boot. Any unsaved data will be lost.
- Potential application consistency issues: Some applications may not function correctly after a revert if memory wasn’t captured, especially if disk write operations were incomplete at the time of the snapshot.
When should you capture memory?
- Testing and Development: When you need to revert a VM to a specific running state for testing or debugging.
- Strict application consistency requirements: For critical applications like database servers that need application-level consistency guarantees.
- Before upgrades/patches, needing quick rollback to running state: To ensure you can quickly revert back to the pre-operation state.
When should you avoid capturing memory?
- Regular backups: Capturing memory is typically not necessary for routine backups.
- Performance-sensitive environments: When the performance impact of capturing memory is unacceptable.
- Limited storage space: When snapshot file size is a major constraint.
- VMs with very large amounts of RAM: Capturing memory on large-memory VMs can take a significant amount of time.
Conclusion:
The decision of whether or not to capture memory depends on your specific needs and environment. Weigh the pros and cons carefully and choose based on your individual use case. In the vSphere Client, you can enable or disable memory snapshots by selecting or deselecting the “Snapshot the virtual machine’s memory” checkbox.
Recommendation: If you’re unsure, start by taking snapshots without memory. If you encounter issues with application consistency after reverting, then consider taking snapshots with memory. You can also look into using the quiescing option, which utilizes tools like VSS (Volume Shadow Copy Service) to ensure application consistency, which can be an alternative to memory snapshots in some scenarios.
