This content is part of the Essential Guide: A complete guide to XenApp and XenDesktop vs. Horizon

How to use Citrix AppDisk for application layering

With Citrix AppDisk, XenApp and XenDesktop customers have another tool for delivering virtualized applications to users. IT administrators might also be able to reduce the amount of time they spend managing base images.

Citrix AppDisk, a new application layering technology in XenApp and XenDesktop 7.8, allows IT to package and deliver apps separately from golden images.

Citrix AppDisk helps to mitigate some of the challenges of managing complex application deployments, such as creating multiple golden images to accommodate different departments or types of users. Traditionally, each image contains the operating system (OS) and applications necessary to meet a user, group or department's specific needs. As an organization's XenApp/XenDesktop deployment grows larger and more complex, so do the number of images and the maintenance time to support them all.

The basics of Citrix AppDisk

AppDisk allows IT to create, deploy and update fewer master images. Instead of installing applications alongside the OS, IT can package and deliver them separately in departmental AppDisks. An AppDisk is either a virtual hard disk or virtual machine disk file, depending on the hypervisor -- either Citrix XenServer or VMware ESXi.

No app layering product is without issues and limitations.

The virtual disks that house the applications act as an independent storage layer that attaches to the base golden image when users boot a XenApp or XenDesktop session. The hypervisor controls the process of attaching the AppDisk's drives to the base image. From the users' perspectives, the apps contained in the AppDisks perform as if they are installed locally on the virtual machine (VM).

AppDisk applications can communicate with each other to support common operations, just like locally installed applications. This behavior is much different from app virtualization platforms, such as Microsoft App-V, which usually isolate the applications from one another. At the same time, the layered applications remain independent of the golden image, making it easier to implement and manage them.

To deploy an AppDisk, IT can use either Citrix Provisioning Services or Citrix Machine Creation Services. Setting up an AppDisk drive involves several basic steps. IT must first create the AppDisk image, install one or more applications and then seal the image. From there, an administrator adds the AppDisk image to a delivery group, which controls who can access those applications.

Citrix AppDisk is just one several features and products conferencegoers can learn about at Synergy 2016, which takes place May 24-26 in Las Vegas.

AppDisk rules of the road

Citrix AppDisk is only available in XenApp and XenDesktop 7.8. Customers cannot purchase AppDisks as a standalone product or use it in earlier versions of XenApp or XenDesktop.

IT can only attach an AppDisk drive to a VM at boot time, not while the VM is running. In addition, AppDisks are currently only compatible with Windows-based VMs, which means no AppDisks on Linux. IT can assign the same AppDisk to multiple base images, however, even if they're running different versions of Windows -- both the desktop and server editions.

There are a few caveats. The Windows environments must all be the same bit size -- 32-bit or 64-bit -- and the applications within the AppDisk must support the version of Windows that IT wants to attach it to. IT can choose which version of Windows to base an AppDisk on, but attaching that AppDisk to a base image running an earlier version of Windows might cause compatibility issues. For example, AppDisks based on Windows 10 might run into issues on a VM running Windows 7.

There is no hard limit on the maximum number of AppDisks an organization can attach to a single VM, but IT should consider its resources and hypervisor limitations. Citrix recommends grouping together related applications into a common AppDisk to minimize the overall number of layers.

Citrix AppDisk drawbacks

Citrix seldom mentions AppDisk without also throwing in a plug for AppDNA, a XenApp and XenDesktop Platinum edition feature that identifies and, if possible, resolves app compatibility issues. If AppDNA can't fix a compatibility issue, it provides step-by-step remediation guidance. AppDisk's integration with AppDNA is one of the features that helps it stand out from other application layering products. Otherwise, Citrix is mostly keeping pace with other vendors, such as VMware and its App Volumes offering. The snag is that some organizations cannot afford XenApp and XenDesktop Platinum.

One of the biggest challenges of using AppDisk is updating applications. AppDisks are read-only, sealed packages. To update an application, IT essentially has to clone the AppDisk image, apply any updates, and deploy a new AppDisk, which does not attach to users' virtual desktops until they reboot.

Additionally, not all applications are well-suited for app layering. Any app that requires its drivers to load before the AppDisk drivers, such as antivirus software, needs to be installed on the base image.

AppDisk currently does not support Microsoft Hyper-V or Nutanix Acropolis hypervisors. In addition, because AppDisks are read-only, they cannot collect user profile data. With that in mind, no app layering product is without issues and limitations.

Before an organization commits to using AppDisks, it should weigh whether app layering solves any of the VDI issues it's running into. For instance, if app layering doesn't significantly reduce the number of golden images IT has to manage, it might not be worth the effort to implement an entirely new technology. Application layering can be a useful tool for some situations, but that doesn't make it a panacea for all app management challenges.

Next Steps

An overview of app layering

How VMware App Volumes works

Learn what's ahead at Citrix Synergy 2016

Dig Deeper on Citrix virtual desktops