🧠 VMware Snapshots — The Complete Deep Dive
Snapshots are one of VMware’s most powerful yet misunderstood features.
They let you capture a VM’s exact state (disk, memory, and config) and return to it later.
But they also impact performance and datastore health if used carelessly.
This post explains — in detail — how snapshots work, what happens during revert, OS impact, and cluster-level risks.
⚙️ 1. What Is a VMware Snapshot?
A snapshot preserves a VM’s disk, memory, and power state at a point in time.
After it’s taken:
The base disk becomes read-only.
All new writes go to a delta disk.
Optionally, memory and CPU state are saved too.
🧩 2. Files Created During a Snapshot
File Purpose
Base Disk (.vmdk) Original virtual disk, becomes read-only.
Delta Disk (-delta.vmdk) Stores changes after the snapshot.
Memory File (.vmem) Captures RAM contents if “snapshot memory” is enabled.
Snapshot Metadata (.vmsn) Records configuration, disk, and memory references.
🔍 3. How Snapshot Works
- Creation
VMware freezes disk I/O briefly.
A new delta file (vmname-000001-delta.vmdk) is created.
Writes now go to the delta file, keeping the base disk intact.
- Retention
Each snapshot adds another delta file, forming a chain.
Reads span across all deltas and the base disk.
- Deletion (Commit)
Changes in delta files are merged back into the base disk.
Deletion can trigger heavy I/O depending on delta size.
🔄 4. What Happens During Snapshot Revert
- Disk State
VMware reconstructs the snapshot point by combining the base and snapshot delta.
The VM now reads from the reconstructed snapshot state.
- Memory State
If memory was captured, .vmem restores RAM and CPU registers.
Processes resume exactly as they were — no reboot.
- OS Behavior
The OS is not rebooted, but uptime resets if memory is restored.
Some sessions may drop briefly, but the VM remains reachable.
⚠️ 5. Why OS Takes Time to Stabilize After Revert
Even if the vSphere task shows “Revert completed”, the guest OS may need minutes to recover.
That’s because:
Disk caches, journaled filesystems (ext4/NTFS), and swap files revalidate.
VMware triggers background I/O to reattach or consolidate delta data.
This causes temporary CPU and I/O spikes until the OS stabilizes.
🧠 6. Why Delta Files Are Needed
Even when reverting to the base disk, VMware must read delta files because:
They contain changed blocks since the snapshot.
To restore the exact state, VMware applies those deltas backward.
Hence, deltas remain essential even when reverting to “base.”
📁 7. .vmsn and .vmem Explained
File Description
.vmsn Snapshot descriptor containing VM config, disk, and memory pointers.
.vmem Memory dump used to resume the VM’s running state instantly.
🧱 8. What You Can’t Do While Snapshots Exist
Snapshots freeze certain VM operations. You can’t:
Change hardware version.
Expand disks or modify RDM mappings.
Add or remove virtual disks.
Convert the VM to a template (in some cases).
These are blocked to maintain snapshot integrity.
🧮 9. Uptime, Reachability & Performance
Uptime Reset: If memory was saved, uptime reverts to snapshot time.
Reachability: Minor drop during revert; VM becomes accessible soon after.
Performance: Expect short-lived I/O spikes post-revert.
Duration: Snapshot and revert times scale with VM disk size and snapshot depth.
⚡ 10. Speeding Up Snapshots and Reverts
By default, VMware snapshots all attached disks, slowing large VMs.
✅ Pro Tip
If a VM has large, static disks (e.g., archives or NFS mounts):
Temporarily detach those before taking or reverting a snapshot.
Only attached disks are processed, reducing time drastically.
⚠️ Caution
Never detach disks with OS mounts or active apps.
Always reattach using the same SCSI IDs after the operation.
🧩 11. Managing Snapshots in vSphere
A. Take a Snapshot
Right-click VM → Snapshots → Take Snapshot
Name it (e.g., PrePatch_2025-10-15).
Check:
✅ Snapshot the VM’s memory
✅ Quiesce guest file system
- Click OK
B. View Snapshots
Right-click VM → Snapshots → Manage Snapshots
C. Revert
Select snapshot → Revert to Snapshot
Wait for the OS to settle before use.
🧹 12. Best Practices
Avoid keeping snapshots >72 hours.
Consolidate or delete snapshots regularly.
Monitor datastore space — deltas grow fast.
Verify app health after revert.
Never use snapshots as backups.
⚠️ 13. When Snapshots Grow Too Large — Cluster-Wide Impact
- What Happens When They Accumulate
Each snapshot creates a delta that grows with every write.
VMware must traverse all deltas to read a block — adding latency.
Long snapshot chains cause severe disk I/O degradation.
- VM-Level Impact
Slower I/O and degraded performance.
Long consolidation (merge) times.
Backup jobs slow down or fail.
Datastore fill-up can pause or crash VMs.
- Cluster-Level Impact
Datastore Pressure: Deltas consume vast space.
vMotion Failures: Large chains increase transfer time.
I/O Spikes: Snapshot consolidations trigger datastore storms.
vSAN Issues: More objects and resync operations, slowing cluster balance.
- Prevention
Automate snapshot cleanup with vCenter alarms or scripts.
Monitor datastore usage.
Keep chain depth ≤ 2–3.
Schedule consolidations during off-hours.
Use backup tools that auto-remove snapshots.
- In Short
Large snapshots are silent datastore killers.
The more deltas you keep, the slower your VMs — and the riskier your cluster.
Consolidate early, consolidate often.
🧾 14. Summary
Area Key Point
Snapshot Role Point-in-time rollback for quick recovery or testing
Delta Files Hold all post-snapshot changes
Revert Restores disk/memory state without reboot
OS Impact May pause briefly as background I/O completes
Performance Tip Detach static disks for faster snapshot ops
Cluster Risk Large deltas impact datastore and vMotion
Best Practice Keep snapshots short-lived and managed
✍️ In Short
VMware snapshots are like time machines — powerful but costly.
Every revert, merge, and delta read adds I/O overhead.
Use them wisely, monitor size, and let the OS stabilize before declaring success.
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