The Great Debate: Persistent vs. Non-Persistent Virtual Desktops Persistent desktops that sustain all user and IT customizations? Or non-persistent desktops that revert back to a pristine state after each use? This has been one of the great debates in Virtual Desktop Infrastructure (VDI). Is it even relevant anymore? Find out in this white paper.
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The Great Debate: Persistent vs. Non-Persistent Virtual Desktops Persistent desktops that sustain all user and IT customizations? Or non-persistent desktops
that revert back to a pristine state after each use? This has been one of the great debates in
Virtual Desktop Infrastructure (VDI). Is it even relevant anymore? Find out in this white paper.
Introduction
The debate between persistent and non-persistent virtual desktops has been hotly contested ever since Virtual Desktop Infrastructure (VDI) became a viable alternative to physical PCs. This white paper takes a look at the history and evolution of the VDI market to understand why persistent and non-persistent desktop models exist.
First, we will define each model, explain the use cases, and discuss the technologies available to implement both desktop types.
Then, we will debunk some of the common myths and misconceptions, and outline the pros and cons of each model.
Throughout, you will benefit from actionable insights that can be used when architecting your VDI environment, and learn from the real-life customer examples that have been included for reference.
Evolution of Persistent and Non-Persistent Desktops
To understand why there are even two different types of virtual desktop models, it is important
to understand how the VDI market and the technology have evolved over time.
The Persistent Desktop Era
Early adopters of VDI chose to virtualize desktops primarily for the security, mobility, and
anytime/anywhere access benefits. The easiest and fastest way to realize these benefits was to
simply move PCs from the edge into the data center.
The first virtual desktops, then, were “full clones” – full-sized virtual machine (VM) copies of
physical desktops. These full clones were assigned at login to specific users. Every time users
would login, they would access the same VMs. Full clones were also persistent – every change
made to desktops by an end user or an IT administrator was saved.
Full clone desktops were managed exactly the same as physical PCs – using manual methods, or
agent-based PC lifecycle management software such as Microsoft System Center Configuration
Manager (SCCM), Norton Ghost, KACE, LANdesk, Big Fix, etc.
Full-sized, thick-provisioned VMs proved to be quick to implement. However, they had two
significant problems that limited broad VDI adoption:
1. Management. The management challenges associated with physical PCs still remained.
Like physical PCs, persistent desktops would deteriorate over time, as IT and end users
both made changes to the C: drive and Windows registry. As desktops deviated from
each other, Windows and application updates wouldn’t always “take,” resulting in the
same 5-15% patch failure rate and costly service desk escalations as PCs. Provisioning
new desktops and delivering new applications was only marginally better than PCs.
2. Storage. Persistent desktops made VDI unaffordable to the mass market due to the
high cost of shared storage. The 40 GB local disk drive in a PC is cheap. That same 40
GB desktop hosted on SAN storage in the datacenter is expensive – roughly 10X the cost
of PC storage.
Because of these issues, only organizations with large IT staffs and large budgets (e.g. Wall
Street financial firms) were able to deploy persistent VDI at any scale.
The Non-Persistent Desktop Era
In an attempt to solve the storage and management challenges, Citrix and VMware introduced
the concept of non-persistent, floating desktops. With this model, virtual desktops would
reside in pools. When a user needed to access a desktop, a VM would be pulled out of the pool
and assigned to the user. When the user was finished using the desktop, it would be returned
to the pool and all changes made by the user would be thrown away – hence the term “non-
persistent desktop.”
It was hoped that this model would address the two challenges of persistent desktops:
1. Management. By resetting desktops back to their pristine state after each use and
using shared image technology to provision desktops, management is greatly simplified.
Disk data is streamed dynamically and in real time from a single shared image, providing
machine image consistency and enabling large pools of desktops to completely change
their configuration, applications, and even OS in the time it takes them to reboot.
Because all desktops are bitwise compatible, the 5-15% patch failure rate vanishes, and
support costs are greatly reduced.
2. Storage. Since no desktop changes would be saved, block-based image sharing
technology could be applied to reduce storage.
VMware already owned an image
sharing technology called Linked
Clones, which had been used for many
years to reduce the hardware costs of
using VMware Workstation.
VMware integrated Linked Clones with
VMware View Composer and
introduced it to customers as the
preferred method of provisioning
desktops for VMware Horizon View.
With Linked Clones, the cloned desktop
uses the virtual disk of the parent
virtual machine from which it was
cloned. This dramatically reduces the
time needed to set up a virtual
machine, and the amount of disk space
the clone uses.
When the desktop is created as a
Linked Clone, a Delta Disk is "linked" to
the replica disk. The Delta Disk is where
all changes or differences between the parent VM and the cloned desktop are stored.
Citrix also owned shared image
technology. Citrix Provisioning Services
(PVS) has been used for years to manage
terminal server farms. It was a natural
step for Citrix to make PVS a provisioning
option for Citrix XenDesktop.
Like View Composer and Linked Clones,
PVS shares a single shared disk image
(vDisk) rather than copying full images to
individual machines. This enables
organizations to reduce the number of
disk images that they manage.
PVS uses a cache file to capture the
changes to the underlying OS when
running in "Standard" mode. There is
also a "Private" mode that enables you
to save changes to a vDisk in the disk
itself.
Generally, administrators use two vDisk
images for a build. One vDisk image is in
standard mode streaming to the desktops. The other vDisk image is updated for new revisions.
The two vDisk images either have to be kept in sync, or clones need to be made each time an
update is performed. For VDI, this process can be tedious compared to other solutions, but the
ability to use either local storage or SAN storage is a huge benefit.
Citrix also offers a similar image sharing technology called Machine Creation Services (MCS) for
customers who want to provision only VDI desktops.
Early VDI adopters were eager to implement the non-persistent virtual desktop model because
of its potential to reduce storage costs and simplify image management. It proved adequate for
users who required the same applications and who didn’t mind that their customizations were
lost after each use. Call centers, kiosks, and static labs that rarely changed were ideal use
cases.
However, these use cases accounted for only 5-7% of all desktops. For end users who expected
their virtual desktops to act like PCs and preserve settings and user-installed applications, and
for IT administrators who needed to frequently reconfigure desktops to meet the needs of
different departments, use cases, projects, and business units, non-persistent desktops proved
inadequate.
The “Non-Persistent Desktop Plus Point Tool” Era
To address the need for desktop customization and personalization while retaining the efficient
resource utilization and single image patching benefits of non-persistent desktops, Citrix and
VMware embarked on strategies to integrate and/or acquire third-party solutions. By adding
profile management, user virtualization, and application virtualization technologies on top of
their existing non-persistent desktop models, customers would – in theory – get everything
they needed.
User virtualization tools would capture and
restore the user-installed applications, IT-
installed one-off applications, and application
plug-ins and add-ins that live outside of a user
profile, and therefore cannot be captured by
profile management technology. Citrix acquired
RingCube, whose technology became Citrix
Personal vDisk.
Profile management tools would capture and
restore the user settings and customizations that
were lost when non-persistent desktops were
reset. VMware acquired RTO Software, whose
technology became VMware View Persona.
Citrix acquired Sepago, whose technology
became Citrix User Profile Management.
Application virtualization tools would enable
different applications to be delivered to desktops
based on the needs of each user by separating
the applications from the shared
base/parent/master image. VMware acquired
Thinstall, whose technology became VMware
ThinApp. Citrix recommends Microsoft App-V
through its strategic relationship with Microsoft.
Image management and storage optimization
tools would enable desktops to be provisioned
from a common base image to reduce storage
costs and simplify patch management. VMware
combined Linked Clones with VMware View
Composer. Citrix integrated PVS with Citrix XenDesktop, and later launched Machine
Creation Services (MCS) to offer a simpler provisioning solution designed specifically for
VDI.
This model became the most common desktop virtualization model, despite being fraught with
complexity and limitations that will be discussed later.
The Layering Era
More recently, a new, unified solution for
provisioning and managing virtual desktops and
applications has taken hold, replacing the need
for non-persistent desktops managed by point
tools. Hundreds of organizations are now using
desktop layering technology from vendors like
Unidesk to provision persistent and non-
persistent desktops from a single shared
Windows OS layer, any number of shared
Application layers, and unique Personalization
(User) layers.
With desktop layering technology, the old debate of persistent vs. non-persistent desktops
becomes obsolete. Layered desktops can be persistent or non-persistent. Yet, both types of