Understanding persistent vs. nonpersistent VDI

When it comes to virtual desktop infrastructure, administrators have a lot of choices. As a virtual desktop admin, you may have wondered about the differences among VDI software options, remote display protocols and all the licenses out there. In this series, we tackle some of the biggest head-scratchers facing VDI admins to help you get things straight.

There are two main types of desktops you can deploy in a virtual desktop infrastructure (VDI): persistent and nonpersistent. So what's the difference?

What is persistent vs nonpersistent VDI?

With persistent VDI, each user gets his or her own persistent virtual desktop -- also known as a one-to-one ratio. Nonpersistent desktops are many-to-one, meaning that they are shared among end users. Each setup has its advantages and disadvantages when it comes to storage, management and customization, so you need to know how each environment works. Let's get this straight.

Persistent VDI

With one-to-one persistent VDI, each persistent desktop runs from a separate disk image. The user's settings are saved and appear each time at login. These types of desktops allow for more Personalization, but they require more storage and backup than nonpersistent desktops.

Pros: Customization and familiarity

It's easier to personalize persistent desktops because users can access their own data, shortcuts and files from the same desktop every time they log in. That aspect of persistent desktops tends to help users embrace VDI more easily because it provides the same consistency and customizations that typical desktops include.

Plus, persistent VDI is basically the same setup you had with your physical desktops, making it easier for many admins to manage. Rather than re-engineering your desktops when you move to VDI, you can stick with a one-to-one setup.

Cons: Storage requirements and image management

Storage is a major concern with persistent VDI. All those individual, customized disk images require more storage capacity than a single golden image does with nonpersistent desktops. The golden image, also known as a master image, is one or several stripped-down desktops that contain only essential applications and data. User settings and customizations are stored separately as user layers that the VDI applies to the golden image desktop during the delivery of the VM.

Storage for persistent desktops is usually a separate logical drive, so it's integrated with the underlying VM, while the actual user data is stored on the desktop itself. Recently, more storage products and features have been made available for persistent desktops, eliminating some of the storage constraints that kept administrators away from persistent desktop virtualization in the past.

An additional concern is that it's more complex to manage numerous diverse images than a master image, which can be altered and updated in one stroke.

Nonpersistent VDI

When users access a nonpersistent desktop, none of their settings or data is saved once they log out. At the end of a session, the desktop reverts to its original state and the user receives a fresh image the next time he logs in.

Pros: Image manageability, greater security, less storage

Since nonpersistent desktops are built from a master image, it's easier for administrators to patch and update the image, back it up quickly and deploy company-wide applications to all end users. Users can't alter desktop settings or install their own applications, making the image more secure. Plus, if the image is hacked or compromised, you can simply reboot desktops back to a clean state. If the same instance happened with a persistent desktop, that desktop user's credentials or other sensitive data could be compromised.

This setup also means there's less storage to deal with. User configuration settings and data are stored on separate hardware that's accessible remotely, such as a network share. That separates the OS from user data and allows admins to store that data on a lower-cost device.

Cons: Less personalization, application flexibility

With nonpersistent VDI, users cannot easily personalize their desktop. That's because nonpersistent desktops don't require individual user profiles; in fact, some organizations deploy nonpersistent VDI so they don't have to manage profiles. You can even configure user profiles to delete themselves automatically from desktops using VMware View or Citrix XenDesktop pools.

Since users share a common disk image, there's a certain amount of desktop customization admins need to ensure users can access all the apps they need. Virtual desktop admins can simply create several golden images for this situation -- one golden image for each type of user or department based on the users' needs. That often requires application virtualization or user environment virtualization, which can get complicated.

IT administrators can look to technologies such as application layering to deliver virtual applications as well. This virtualization technology stores and runs virtual applications in a separate layer from the virtual desktops. However, once a user accesses the VM, the VDI delivers the applications in addition to the desktop. The user can then interact with the layered applications as if they were native to the OS.

However, not all apps lend themselves to being virtualized. Legacy applications, especially, may cause issues when organizations try to virtualize them.