Desktop Virtualization: 5 Most Popular Flavors, Explained

CIO — One of the big questions in technology for the last three years has been how end users will adopt desktop virtualization. The answer, at least from some early adopters, seems to be "how won't we do it?"

A survey released by the Enterprise Management Associates in September found that companies with desktop virtualization projects in place or underway were almost all using more than one method of delivery, ranging from traditional terminal services to server-based applications accessed through a Web browser, according to Andi Mann, VP of research for the Boulder, Colo. consultancy.

[ For timely virtualization news and expert advice on strategy, see CIO.com's Virtualization Drilldown section. ]

A typical Citrix XenDesktop virtual desktop connection takes up between 56Kbit/sec and 100Kbit/sec of network bandwidth, and can satisfy the needs of many users by running shared operating systems and applications on back-end servers, according to George Thornton, network operations manager for Texas; Montgomery Independent School District, which standardized two of the three schools it opened this fall on Citrix virtual desktops.

His users who need more power for graphics-intensive or number-crunching applications can use the same thin-client hardware, but connect to a virtual machine on the server that supports only that one user, along with the additional memory or processing power required. That requires more like 2 megabits of network bandwidth per second, so Thornton kept those connections to a minimum, he says.

With all of the market competition around desktop virtualization, some users are confused about the pros and cons of the various options. Here's a snapshot of the major desktop virtualization approaches and of the types of situations for which they might be appropriate.

1. Remote Hosted Desktops:

What most people think of when they think "terminal services." A server runs one image of an operating system or application and many clients log in to it using connection broker software that is the only part of the software hosted on the client machine. Client machines operate only to show an image on the monitor of the application that user is sharing, and to transmit keyboard and mouse input back and forth.

Advantages: Low cost, high degree of control over data and applications.
Disadvantages: Performance depends on the quality of the network connection; display protocols often can't handle complex graphics; some applications designed for desktops can't run in shared mode on the server; inflexible for end users, who can't store data locally, use most peripherals, or move data back and forth using thumb drives. Does not work when disconnected.

Example vendor offerings:
Software: Citrix XenDesktop; Wyse ThinOS; Microsoft Remote Desktop Services; Microsoft Enterprise Desktop Virtualization (MED-V); VMware View Manager.
Hardware: Pano Logic Device, Remote; nComputing thin clients; Wyse thin clients; Sun Ray Ultra-Thin client; Symbiont Network Terminal; Rangee Thin Client

2. Remote Virtual Applications:

What you get in every Web application you've ever used. Differs from shared desktops in that the only thing required is a browser and standard Web protocols (HTTP, HTTPS, SSL, etc.) to create secure connections and transmit graphics and data. Depending on design of the applications (think Flash downloads) the end-user's machine may process some of the application's logic or graphics, or may only light up the monitor and send clicks to the server.

Advantages: Doesn't require that IT control the hardware or software environment of the end user.
Disadvantages: Doesn't allow IT to control the hardware or software environment of the end user, which could affect performance. Does not work when disconnected.

Example vendor offerings: Citrix XenApps; Microsoft Remote Desktop Services; VMware View; VMware ThinApps.

3. Remote Hosted Dedicated Virtual Desktops:

The next step up in power for end users and step down in cost and resource conservation for IT from Web apps or terminal services. Rather than having many users share one instance of the same application or operating system, the server hosts an entire operating system and set of applications within a virtual machine that is accessible only to that user. The VM could run on a server, sharing resources with other dedicated VMs, or could run by itself on a blade PC. Can either be hosted remotely or streamed. In the streamed scenario, both applications and operating systems can be streamed to the client—downloading parts of the software as the user requires them, and executing on the client machine, using its processing power but not local storage.

Hosted remotely:
Advantages: Can run applications that balk at running in shared mode; isolates activity of each user to prevent resource constraints.
Disadvantages: Uses far more bandwidth than shared desktops, and far more hardware on the server. Performance still depends on the quality of the network connection and ability of the display protocol to handle graphics. Does not work when disconnected.
Example vendor offerings: Citrix XenDesktop; Wyse ThinOS; VMware View; Microsoft Remote Desktop Services; Microsoft Enterprise Desktop Virtualization (MED-V)

Streamed:
Advantages: Often gives the end user better performance because demanding graphic or other operations execute locally.
Disadvantages: Requires more powerful client hardware, reducing the cost benefit of virtual desktops. Does not work when disconnected.
Example vendor offerings: Citrix XenDesktop, XenApp, XenProvisioning; Wyse TCX; VMware View Manager, ThinApps, Composer; Microsoft VDI suite.

4. Local Virtual Applications:

Think "Java." Applications download from the server to the client machine and run there, using local memory and processing power. But they run within a "sandbox" that enforces a set of rules on what the local machine can do and to what it can connect.

Advantages: More computing resources and sometimes better performance than remotely hosted applications; less bandwidth consumption; can be used offline.
Disadvantages: Less control by IT over the hardware and security of the data.

Example vendor offerings: Citrix XenApp, Wyse TCX, VMware ThinApp, Microsoft Application Virtualization.

5. Local Virtual OS:

Present in two major versions. Option one: A client-side hypervisor can create a virtual machine within a laptop or desktop computer, which can function as a completely standalone unit that keeps itself separate from hardware and software on the client machine outside of the VM. Option two: A hypervisor runs on the machine's BIOS, allowing the user to run multiple operating systems with no "host" OS at all.

Advantages: Multiple OSes on a single system; no concerns about OS compatibility, can run on non-traditional VM clients such as smartphones or PDAs.
Disadvantages: Potential conflict for resources, relative immaturity of client-side hypervisors leaves security unproven.

Example vendor offerings: Citrix Dazzle and Receiver, Wyse PocketCloud, TCX, VMware View Client Virtualization with Offline Desktop (Experimental); Microsoft VDI suite.

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