From your VMware vCenter Server (or VCSA), you will be able to create snapshots on your virtual machines to save the state of a virtual machine and then be able to restore it in a few seconds.
You can even save the state of a virtual machine when it is started, if you wish.

1. Snapshot information
2. Snapshot limitations
3. Create a snapshot
   1. Create a cold snapshot
   2. Create a hot snapshot
4. Create a snapshot hierarchy and manage snapshots
5. Consolidate snapshots
6. Impossible to expand a virtual hard disk after creating snapshots

1. Snapshot information

In accordance with VMware KB 1025279 (Best practices for using VMware snapshots in the vSphere environment), here are the recommended practices for using snapshots:

• do not use snapshots as backups. Indeed, each snapshot file contains only the data created after the snapshot was created.
  If you delete the virtual machine's base hard drive, the snapshot will be of no use to you.
• you can use up to 32 snapshots (children of each other), but it is recommended to use a maximum of 2 or 3 snapshots for performance reasons.
• avoid using a single snapshot for more than 72 hours, as the snapshot file may become large.
  This risks impacting the performance of your virtual machine, as well as consuming space on your datastore.
• if you want to use third-party backup software, make sure the snapshots are deleted after a successful backup has been performed on that virtual machine.
• avoid using snapshots if you want to be able to migrate storage from a virtual machine to another host using vMotion. In fact, this feature is only supported since vSphere 5.0.

2. Snapshot limitations

When using snapshots, you will not be able to use:

• of physical disks from the virtual machine
• of iSCSI disks configured in the guest operating system via an iSCSI initiator
• host physical devices via PCI passthrough (vSphere VMDirectPath I/O)
• and more

To learn more about limitations for using snapshots, see the "Snapshot Limitations" page of the official VMware documentation.

3. Create a snapshot

3.1. Create a cold snapshot

To start, you can create a (cold) snapshot of your virtual machine to back up your virtual machine very quickly.
In this case, the virtual machine must therefore be turned off (power off).

To take a snapshot of your virtual machine from the vSphere Client, click on the 5th icon located to the right of your virtual machine name.

Or click: Actions -> Snapshots -> Take Snapshot.

In the "Take Snapshot" window that appears, indicate the name under which you want it to appear in the snapshot management of this virtual machine, then click OK.
If you wish, you can also add a description or comment for this snapshot, but this is optional.

Note: the 2 boxes "Snapshot the virtual machine's memory" and "Quiesce guest file system (Needs VMware Tools installed)" are grayed out, because these options are only available when the virtual machine is started (powered on).

If you look in recent tasks, you will see that a "Create virtual machine snapshot" task has been run.

To view the created snapshot, click: Actions -> Snapshots -> Manage Snapshots.

In the "Manage Snapshots" window that appears, you can see:

• [Virtual machine name]: the base state of the virtual machine.
• [Snapshot name]: name of the snapshot you just created.
  Note that the snapshot icon is entirely blue, because the snapshot was created "cold" (when the virtual machine was turned off).
• You are here: the current state of the virtual machine.

Start your virtual machine.

Make changes to it.
In our case, we have installed several software programs, the icons of which you can see on the desktop.

To restore the last snapshot created, click: Actions -> Snapshots -> Revert to Latest Snapshot.

Note that it is not necessary to manually power off the virtual machine before restoring the desired snapshot.

A "Revert to Snapshot" window appears.
Answer "Yes" to the question "Revert to snapshot [snapshot name]".

If you look in recent tasks, you will see that a "Restore current snapshot" task has been run.

As expected, the software we had installed in this virtual machine has disappeared.

Note that creating snapshots also saves the configuration state of your virtual machine (virtual hardware and settings).
If you had changed the virtual hardware or settings of your virtual machine after creating the snapshot, restoring it will also have undone those changes.

3.2. Create a hot snapshot

If you need to save the state of a virtual machine when it is started (powered on), know that this is possible.
This is called creating a hot snapshot.

To do this, start the virtual machine and launch whatever you want.

Then, while the virtual machine is powered on, click: Actions -> Snapshots -> Take Snapshot.

As you can see, since the virtual machine is booted (powered on), the "Snapshot the virtual machine's memory" box is available and checked by default.
When this box is checked, VMware vCenter Server will also back up your virtual machine's RAM so it can restore its state when you want to restore your virtual machine's state to this snapshot.

Warning : when you want to save the RAM of a virtual machine, it is important not to allocate too much RAM to this virtual machine.
Indeed, the more RAM you have allocated to your virtual machine, the longer it will take to create and restore this snapshot.

If the virtual machine is started (on), but you do not want to save the RAM of the virtual machine (which is not recommended if programs are currently open in it), uncheck the box "Snapshot the virtual machine's memory".
If VMware Tools is running in the guest operating system, unchecking this 1st box will enable the 2nd box.
If you check the 2nd box "Quiesce guest file system (VMware Tools installed)", the guest operating system will be frozen for a brief moment (thanks to VSS on Windows).
This allows the snapshot data to be as consistent as possible and thus avoid any data corruption that would have occurred following the writing of data by a program currently open in the guest operating system.

If you look directly in the recent tasks, you will see that the "Create virtual machine snapshot" task will take a little longer than when the snapshot was created when the virtual machine was stopped.
This is due to saving your virtual machine's RAM.

A little later this task will be completed.

You can shut down the virtual machine if you want.

Now go to: Actions -> Snapshots -> Manage Snapshots.

As you can see, the snapshot icon slightly different from before.
Indeed, the green triangle displayed on the icon of this snapshot indicates that it was created when the virtual machine was started (powered on).
Moreover, when you select it, VMware vCenter Server will tell you that the "Snapshot the virtual machine's memory" option had been activated.

To restore the state of your virtual machine using the "hot" snapshot created previously, click: Actions -> Snapshots -> Revert to Latest Snapshot.

Note that at the moment our virtual machine is stopped (powered off).

Confirm the restoration of this snapshot by clicking Yes.

As expected, the virtual machine is restored to its powered-on state.
For recent tasks, you will see the "Restore current snapshot" task appear.

As you can see, our machine is already started.
In the event that programs were open when the snapshot was created, they will reappear in the state where you left them.