There are a number of considerations to take into account when building out a VDI infrastructure. The host hardware has to be pretty beefy; multiple multicore processors, scads of RAM, and plentiful disk space are absolute necessities. CPU performance and RAM are easy to come by, and while disks are cheap, choosing the correct storage system can make a huge difference on overall VDI performance. Do not scrimp on the storage system. Lots of spindles, the fastest drives you can afford, and fast I/O are paramount. SSD drives are the current speed kings, and if the budget allows, build out your online storage with them. To really scale your storage, you'll want to host virtual disks on fast SAN, NAS, or iSCSI hardware. All of the major virtualization vendors support these storage technologies.
VDI building blocks
Citrix and VMware take very similar approaches to providing a VDI solution. Each vendor has its own bare metal, or Type 1, hypervisor. Each has its own connection broker to direct incoming user requests to the appropriate virtual disk image. Each provides a browser-based management tool for creating, updating, and managing the virtual desktop images and assigning the virtual machines to users. Each also provides its own remote display protocol: HDX in the case of Citrix, PCoIP in the case of VMware.
Both XenDesktop and View provide the basic types of virtual desktops: dedicated, pooled, and shared, but only XenDesktop can also "stream" a virtual desktop to the end user. Dedicated desktops are stateful virtual machines assigned to specific users, allowing them to customize and preserve their personal settings from session to session. Pooled desktops -- dynamically created from a golden image when users log on, then destroyed when users log off -- are suitable for call centers or sales centers where users perform the same standard tasks and no personal user information is retained.
Shared virtual desktops, also known as session virtualization, are nothing other than Remote Desktop Services (or Terminal Services) sessions. And lastly, streamed desktops -- where client systems boot from server-based desktop images over the LAN -- combine the management benefits of VDI with the performance benefits of client-side execution. Again, only Citrix supports desktop streaming.
Both XenDesktop and View also support "offline mode" -- a form of desktop virtualization that doesn't require a connection to the VDI server farm. Offline mode allows users to download the virtual desktop to their laptop and run it locally. Whenever the user is connected to the corporate network, any changes IT makes to the master image are pushed out to the local virtual machine. And depending on the personalization policy, any changes users make to their desktop are synchronized back to the data center. This mode of operation is aimed at users who are not always in communication via the Internet or corporate LAN.
XenDesktop and View differ little in overall functionality. Their differences fall mainly in two areas: hypervisor support and connection protocol. Citrix built XenDesktop to run on any of the three most popular hypervisors: XenServer, vSphere, and Microsoft Hyper-V. On the other hand, View is tightly integrated with vSphere and doesn't support any other platform.
While both products support Microsoft RDP (Remote Desktop Protocol), each has its own proprietary remote access protocol. Citrix's HDX (High Definition Experience) protocol is TCP-based and includes a slew of network-aware tuning features that helps to improve the remote user experience regardless of the connection quality. VMware's PCoIP (PC over IP) is a UDP-based protocol that is also designed to provide an excellent user experience with less protocol overhead. Both HDX and PCoIP are tremendous technologies in their own right. Arguments can be made for why one is better than the other; suffice it to say that both HDX and PCoIP do a great job of providing high-quality video, audio, and complex graphics -- including Windows 7 Aero -- to the end-user's device.
Through Citrix's proprietary HDX protocol, XenDesktop delivers exceptional performance regardless of connection speed. During my testing with HDX, I connected into my virtual desktops both locally and from outside the network walls. With HDX, I didn't notice any appreciable lag in video or audio to my client. Even when viewing video on YouTube from a remote client, playback and audio quality were excellent.
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