7.7 Virtual Machine Snapshots & VM Creation Methods
Key Takeaways
A snapshot preserves a VM's disk state, settings, and optionally its memory; new writes go to a delta disk, so snapshots are not backups on their own.
Broadcom KB 1025279 says a chain supports at most 32 snapshots, but recommends using only 2-3 and keeping any snapshot no longer than 72 hours.
Quiescing the guest file system requires VMware Tools and is available only when the snapshot does not include the VM's memory.
Snapshots slow a VM down by adding delta-disk overhead, longer read paths, and extra storage use, and deleting or consolidating them causes I/O and brief stuns.
VMs can be created with the New Virtual Machine wizard, by cloning, from a template, by deploying an OVF or OVA template, from a content library, or by registering an existing .vmx file.
7.7 Virtual Machine Snapshots & VM Creation Methods
What a Snapshot Captures (7.1)
A snapshot preserves a VM's state at a point in time:
- Disk state: each virtual disk gets a delta disk (
<vm>-000001.vmdk). From then on, writes go to the delta and the base disk becomes read-only. - Settings: the VM configuration at that moment.
- Memory (optional): if Snapshot the virtual machine's memory is selected for a powered-on VM, its RAM is saved so a revert returns to the exact running state.
| File | Contents |
|---|---|
<vm>-00000N.vmdk | Delta disk. On VMFS-6 it uses the SEsparse format; vSAN OSA uses its own vsanSparse format, and vSAN ESA uses native snapshots |
<vm>.vmsd | Snapshot metadata database: the tree and the names |
<vm>-SnapshotN.vmsn | Snapshot state, including the configuration |
<vm>-SnapshotN.vmem | Memory image, present only when memory was captured |
Snapshot Options
| Option | Meaning | Constraint |
|---|---|---|
| Include memory | Captures running state; a revert resumes the powered-on VM | Takes longer and pauses the VM briefly while memory is written |
| Quiesce guest file system | VMware Tools flushes the guest's file systems (VSS on Windows) for a consistent disk state | Requires VMware Tools, and only applies when memory is not included |
Independent disks are excluded from snapshots. Independent-persistent disks keep all changes, and independent-nonpersistent disks discard changes at power-off.
Snapshot Manager Operations
Open Actions > Snapshots > Manage Snapshots:
- Revert to: returns the VM to the selected snapshot. Changes since then are discarded.
- Delete: removes a snapshot point by merging its delta into the parent. The data is kept; only the restore point disappears.
- Delete All: merges every delta into the base disk.
- Consolidate: when the vSphere Client shows "Virtual machine disks consolidation is needed" (for example after a backup job left deltas behind), run Snapshots > Consolidate to merge the leftover redo logs.
Limits and Best Practices (KB 1025279)
The exam guide references Broadcom KB 1025279, Best practices for using VMware snapshots in the vSphere environment:
- A snapshot chain supports at most 32 snapshots, but use only 2-3 for better performance.
- Do not keep a snapshot longer than 72 hours. Delta disks keep growing, up to the size of the base disk.
- Snapshots are not backups. The delta depends on the base disk, so losing the base loses everything. Backup tools take a snapshot, copy the data, and then delete it.
- Delete snapshots before large changes such as disk expansion or hardware upgrades.
- Some configurations, such as physical-compatibility RDMs, cannot be snapshotted, and Fault Tolerance VMs must have their snapshots removed before FT is turned on.
Performance Impact (5.7)
| Effect | Why |
|---|---|
| Slower writes | New blocks are allocated in the sparse delta disk as the VM writes |
| Slower reads | Reads may have to walk the chain from the newest delta back to the base disk |
| Storage growth | Each delta can grow to the full size of its disk, and memory snapshots add a file the size of the VM's RAM |
| Stuns | Creating a snapshot, especially with memory, and deleting or consolidating one briefly pause the VM; long chains mean longer consolidations |
| Backup side effects | Failed backups can leave orphaned deltas that need consolidation |
Array- or platform-native snapshots reduce these costs: vVols offload snapshots to the array (Section 3.3), and vSAN ESA uses native log-structured snapshots (Section 4.1).
Ways to Create a VM (7.2)
| Method | When to Use It |
|---|---|
| New Virtual Machine wizard | Build a VM from scratch and install the OS from an ISO |
| Clone an existing VM | Copy a configured VM, optionally with a guest customization specification |
| Clone or convert to template, then deploy from template | Standard golden images; apply a customization specification at deployment (Section 7.4) |
| Deploy OVF Template | Import a vendor appliance or exported VM from an OVF package (.ovf descriptor + .vmdk + .mf manifest) or a single-file OVA, from a local file or URL |
| Content library | Deploy VM templates or OVF templates from a local or subscribed library (Section 7.4) |
| Register VM | Add an existing .vmx file from a datastore to the inventory, for example after a recovery |
| Instant Clone (API or PowerCLI) | Create many VMs quickly from a running parent that share its memory and disk state |
| vCenter Converter | Convert physical machines or other hypervisors' VMs (Section 1.7) |
Deploy OVF Template validates the package (manifest checksums and any signing certificate), then asks for the target, network mapping, disk format, and any appliance properties (such as IP settings) defined in the OVF.
Scenario: "Consolidation Needed" After Backups
Problem: Several VMs show Virtual machine disks consolidation is needed, and their datastore is filling up. The backup product runs nightly.
Diagnosis:
- A backup job created a snapshot, then failed or timed out before deleting it cleanly, leaving delta disks that Snapshot Manager no longer lists.
- The backup proxy may still have the VM's disks hot-added, which holds file locks.
Fix:
- Make sure the backup proxy has released the disks (remove any hot-added disks from the proxy VM).
- Run Snapshots > Consolidate on each VM.
- If consolidation fails with a lock error, find the host holding the lock with
vmkfstools -D(Section 8.4) and clear it. - Afterward, confirm there are no
-00000N.vmdkdeltas left in the VM folder and that the datastore's free space recovers.
Prevention: Monitor for VMs that need consolidation, for example with an alarm or a report, and keep backup software and VMware Tools current.
An application team keeps several VM snapshots for weeks as rollback points. Following Broadcom's snapshot best practices (KB 1025279), what should the administrator recommend?
Keep up to 32 snapshots per VM indefinitely, since vSphere supports 32
Convert the snapshots to independent-persistent disks
Use snapshots only while every snapshot includes memory
Use no more than 2-3 snapshots in a chain, keep none longer than 72 hours, and rely on real backups for long-term restore points
An administrator takes a snapshot of a running Windows VM and wants the disk state to be application-consistent through VSS. Which option should be selected, and what does it require?
Quiesce guest file system, which requires VMware Tools and is used when the snapshot does not include memory
Snapshot the virtual machine's memory, which requires a vTPM
Independent-nonpersistent mode, which requires VMFS-6
Consolidate, which requires Storage DRS
A security vendor ships its virtual appliance as a single .ova file. How should the administrator bring it into vSphere?
Register the .ova file as a VM from the datastore browser
Use the Deploy OVF Template wizard and select the .ova file (or its URL)
Upload the .ova into the Product Locker
Convert it with vLCM image export
Sections you finish are checked off in the contents.