Demystifying storage and virtualization snapshots

Vendors have different definitions for storage and virtualization snapshots. Take the time to learn the differences, because they can be a lifesaver.

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Storage and virtualization snapshots are an IT administrator’s best friend. But one of the occupational hazards of talking about “snapshots” in virtualized environments is that just about every storage vendor appears to have different take on what the term means. The situation gets even more confounding when virtualization vendors such as VMware, Microsoft and Citrix tout their own snapshot technologies that work differently than those...

from storage vendors.

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With that disclaimer in place, I do think it is possible to define snapshots. A snapshot is like taking a photograph of your storage. This holds true whether you’re talking about a snapshot at the storage level or the virtualization level. The big difference between storage and virtualization snapshots is how you use themthe practicalities and best practices.

You can configure an array-based system to take a “photograph” of your data every 15 minutes. These photographs simply store the differences accrued from one 15-minute period to the next. After an hour, you would have the original LUN or volume plus four snapshots. This would allow you to go back in time to different states of the volume.

In the virtual world, most vendors require admins to take their virtualization snapshots manually. This means unless the virtualization admin either deletes the snapshot, which merges the changes occurring into the virtual disk, or reverts the snapshot in a process going back in time, then the snapshot “delta”—or difference—will simply grow over time.

Why use storage and virtualization snapshots?
The benefits of snapshots either at storage array level or virtual machine (VM) level are clear. With array-based snapshots, you can roll back terabytes of data in the event of, say, widespread corruption or a virus attack. Snapshots form the bedrock of most storage array replication, so they can be shipped across the wire from one array to another.

That means if a storage array blows up, you would be up and running very quickly. Many storage vendors have integrated this snapshot functionality into virtualization management tools so that is easy to mount a snapshot to the hypervisor and use it to copy a VM that has been accidentally deleted. All this is much quicker than resorting to conventional backup technologies.

The primary use for virtualization snapshots is for VM backups. The virtualization snapshot will normally have application awareness commonly leveraging Microsoft VSS to quiesce the data before the backup of the virtual disks is triggered.

Snapshots are also used to unlock the virtual disk in the virtualization vendor’s files system before being shipped elsewhere. There’s also a day-to-day admin use of snapshots that can be useful.

It’s common for virtualization admins to take a snapshot of a VM before doing a major update or reconfiguration. With the virtualization snapshot, the admin is empowered to revert the state of the VM without having to interact with the storage team.  

In many ways, virtualization snapshots can act like a giant “undo” button that allows admins to undo a task but without any of the granularity of a Microsoft Office application.

Although they have their advantages, storage and virtual machine snapshot challenges can arise.

This was first published in March 2012

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