VMware Snapshot Overview

This post will talk about how VMware snapshots work, what they should and should not be used for, and provide a demonstration. A snapshot preserves the state and data of a virtual machine from a specific point in time. You can create multiple snapshots to save the virtual machine in different stages of a work process. Snapshots are managed using Snapshot Manager in the vSphere web client, or with PowerCLI. You should not manually alter any of the snapshot files as this may compromise the disk chain, with potential for data loss.

What happens when I take a snapshot?

When you take a snapshot of a virtual machine a number of files are created; a new delta disk (or child disk) is created for each attached disk, in vmdk format. The delta disks follow a naming convention and sequence of vmname-000001.vmdk, vmname-000002.vmdk and so on. These files are stored with the base vmdk by default. Any changes to the virtual machine are written to the delta file(s), preserving the base vmdk file. Think of this delta file as a change log, representing the difference between the current state and the state at the time the snapshot was taken. A .vmsd file is created to store the virtual machine snapshot information defining the relationships between child disks. A .vmsn file and corresponding .vmem file is created if the active state of the virtual machine memory is included in the snapshot. These configuration files are all stored in the virtual machine directory.

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When should I use a snapshot?

Use a snapshot as a short term restore point when performing changes such as updating software versions or for testing software or configuration with unknown effects. You can create multiple snapshots of a virtual machine; VMware recommend no more than 32 snapshots in a chain, however best practise for performance is to keep it low, i.e. 2-3 snapshots.

Do not use a snapshot as a backup. Although it provides a restore point a snapshot relies on the base disk(s), without this the snapshot files are worthless. If you need a restore point for more than a few days then consider other options such as traditional backup, or cloning the virtual machine. According to vSphere best practises a single snapshot should not be used for more than 24 – 72 hours. There are a number of factors that determine how long a snapshot can be kept, such as the amount of changed data, and how the application will react to rolling back to a previous point in time. Some disk types and configurations are not supported by snapshots, you can see a full list of limitations here.

What are the risks of using a snapshot?

The more changes that are made within the virtual machine the more data is written to the delta file. This means the delta file grows quickly and in theory can grow as large as the virtual disk itself if the guest operating system writes to every block of the virtual disk. This is why snapshots are strictly a short term solution. Ensure there is sufficient space in the datastore to accommodate snapshots, if the datastore fills up any virtual machines residing in that datastore will be suspended.

How do I take a snaphot?

From the vSphere web client right click the virtual machine to snapshot, select Snapshots, and Take Snapshot. Note that vCenter Server is not a requirement, snapshots are also supported through the local ESXi host web UI.

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Enter a name and description for the snapshot. The contents of the virtual machines memory are included in the snapshot by default, retaining the live state of the virtual machine. If you do not capture the memory state, then the virtual machine files require quiescing, otherwise should the virtual machine be reverted to a previous state; then the disks are crash consistent. The exception to this is taking a snapshot of a powered off virtual machine, as it is not possible to capture the memory state, or quiesce the file system.

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To view active snapshots locate the virtual machine in the vSphere web client and select the Snapshot tab. Snapshots are listed in order with ‘you are here’ representing the current state, at the end of the snapshot chain.

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It is possible to exclude disks by changing the disk mode to independent, covered here. However please use this option with care as it may have other implications. For example if your backup software uses snapshots as part of the backup process then setting independent disks may inadvertently exclude these disks from backups.

 How do I revert back to a snapshot?

Select the snapshot you want to revert back to, and click the revert icon in the top left of the snapshot menu. The icon dialog reads ‘revert the VM to the state it was in when the snapshot was taken’.

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Review the confirmation message. The virtual machine state and data will be reverted back to the point in time when the selected snapshot was taken. The current state of the virtual machine (changes made since the snapshot was taken) will be lost unless you have taken a further snapshot. Click Yes to continue.

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If you have multiple snapshots you will see the ‘you are here’ marker move to the point in the chain you have reverted to. Snapshots taken after this point are still valid and can be reverted to if required. After you have reverted to a snapshot you are happy with you need to save, or commit, the state of the virtual machine. More on this below.

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How do I keep the state of the virtual machine?

When you keep the current state of the virtual machine the delta disks are merged with the base disks, committing the changes and the current state of the virtual machine. This is done by using the delete snapshot options in Snapshot Manager.

  • Delete All – deletes all snapshots from the virtual machine. This merges the delta disk(s) with the base disk(s) to save, or commit, the virtual machine data and configuration at the current point in time. If you have reverted to a snapshot you still need to delete all snapshots to start writing to the base disk again.
  • Delete – deletes individual snapshots from a chain; writing disk changes since the previous snapshot to the parent snapshot delta disk. If only a single snapshot exists then deleting this snapshot is the same as a Delete All for multiple snapshots; the VM state is committed and data is written to the base disk as normal.

Right click the virtual machine in the vSphere web client and select Snapshots, Manage Snapshots. From the All Actions menu select Delete Snapshot to delete the selected snapshot, or Delete All Snapshots. In this example we are deleting all snapshots, so click Yes to confirm.

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All snapshots are now removed and the current state of the virtual machine is committed to the base disk. Any changes made from here on in are written to the base disk as normal, unless another snapshot is taken.

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What is snapshot consolidation?

Snapshot consolidation is useful if a Delete or Delete All operation fails; for example if a large number of snapshots exist on a virtual machine with high I/O, or if a third party tool such as backup software utilising snapshots is unable to delete redundant delta disks. Using the consolidate option removes any redundant delta disks to improve virtual machine performance and save storage space. This is done by combining the delta disks with the base disk(s) without violating a data dependency, the active state of the virtual machine does not change.

To determine if a virtual machine requires consolidation browse to the vCenter Server, cluster, or host level in the vSphere web client and click the VMs tab. Right click anywhere in the column headers and select Show/Hide Columns. Tick Needs Consolidation and click Ok.

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If a virtual machine requires consolidation right click and select Snapshots, Consolidate. There is also a default alarm defined at vCenter level for virtual machine consolidation needed.

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From vSphere 6 onwards the snapshot consolidation process was improved. You can read more about the specifics, and testing, in this blog post by Luca Dell’Oca.

The snapshot functions described in this post can also be managed using PowerCLI, this blog post by Anne Jan Elsinga covers the commands you’ll need.

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