Snapshots are one of the best features of virtualization technology. With VMware’s vCenter Server, you can create and manage snapshots easily with the controls available. Creating a snapshot and reverting to a snapshot is extremely simple in vCenter, and on more than one occasion has saved me in a pinch.

Snapshots maintain a VMs power state, whether it was powered on or off, and all data including disks, memory and VMs nic configuration. In the event of a bad software install, or unsavory VM change, you can easily revert back to a snapshot in a healthy state.

In cases where more than just the VMware administrator has access to vCenter, the number of snapshots taken and kept can grow over time. Without proper management of snapshots inside of vCenter, disk space can get filled up and the ability to migrate a VM with large snapshots becomes very complicated. Best practices don’t always align with the needs of the organization, however, in this case, it is imperative that snapshot best practices are closely followed.

Using Snapshots as Backups

Virtual machine snapshots are not considered a valid form of backup/disaster recovery. VMware makes it very clear that one should avoid utilizing snapshots for the sole purpose of backups. This is because snapshots can be easily corrupted, and they live on the ESXi host where the VM that you snapshot resides.

If there is a host failure, or if a snapshot becomes corrupt, the “backup” will be lost or unrecoverable. It is better to utilize them as a point in time snapshot of a VMs state. In the event, an install goes awry, or a software test fails, you can revert to the point in time before the changes occurred, utilize a third-party backup program, or rely on other VMware products such as Site Recovery Manager (SRM) and vSphere Replication.

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How Long Snapshots Should Be Kept?

The best practice is not to keep any VMware snapshot for more than 72 hours. The reason behind the 72-hour limit is due to the nature of the snapshot. Snapshots are not meant to be long term backups or fail-safes. Snapshots are meant to be a point in time snaps of a VMs state so that you can revert in the event testing goes wrong or changes don’t take.

In other words, snapshots are a short term solution. Further, keeping a snapshot on disk could potentially create issues when migrations are required, or disk space is running low. If you are an administrator and want to enforce this 72-hour limit, you can easily write a PowerCLI script to delete all VMs older than 72 hours. Clearing up snapshots will improve VM performance and clear up disk space in the datastore.

Controlling the Number of Snapshots per VM

As previously stated, in an environment where multiple types of users have access to vCenter to build VMs and manage their own snapshots, datastore space goes quickly. When multiple users have multiple snapshots on multiple VMs, performance will decline and more storage needs to be provisioned.

Fortunately, there is an easy way to control the number of snapshots allowed per VM. VMware’s best practice for the number of VM snapshots allowed is 32 in a snapshot chain. Any attempt to snapshot a VM’s state beyond 32 in a chain will fail. Regardless, VMs should never come close to having 32 snapshots. A seasoned VM administrator can make a change in the advanced settings of the VM.

Right click on the VM which you intend to edit, and click “edit settings.” The VM settings menu will appear and the option “Edit Configuration…” will be towards the bottom of the window. After clicking on “Edit Configuration…” button, advanced configuration parameters menu will open. Navigate to “snapshot.maxSnapshots” and in the “Value” column, change the default setting to 3. This will only allow a maximum of 3 snapshots for the VM.

Create a Snapshot Policy

Lastly, there are actions that can be taken from enforcing a policy. Proposing a snapshot policy to your management team will help to keep everything clean inside the vCenter environment. A snapshot policy will make a VM administrator’s job much easier at the end of the day. Requiring each person with access to vCenter to sign a snapshot policy will clearly define the rules for taking snapshots, storing them, and deleting them.

In the snapshot policy, it will be explicitly explained that any VMware snapshot older than 72 hours (or whatever threshold management approves) will be automatically deleted and changes will be committed. The script that cleans up old snapshots will do its job and performance won’t decline. Of course, any policy has to be approved by management and enforced. If the policy isn’t enforced, none of the rules will matter and snapshots will wreak havoc on your data center.

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