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What's behind the Citrix XenServer surge?

Research indicates that the XenServer adoption rate outpaces its competition, and users say that XenDesktop drew them to XenServer, but other factors have kept them with Citrix's hypervisor.

While Citrix Systems' share of the server virtualization market lags that of competitors, cost as well as integration with other Citrix offerings may have fostered greater adoption of its XenServer hypervisor.

And according to recent analyst research, purchases of XenServer have grown at a faster rate than those for VMware's vSphere or Microsoft's Hyper-V. Recent XenServer adopters say their deployments usually start with Citrix's XenApp (for application virtualization) or with XenDesktop (for desktop virtualization). But Citrix pricing has expanded XenServer's use, even if it means that users manage multiple "tiers" of hypervisors and dedicate XenServer to lower-tier applications.

According to IDC's market research firm IDC's fourth-quarter 2009 Worldwide Quarterly Server Virtualization Tracker, total VMware licenses increased 19% during that quarter compared with the previous year; Microsoft Hyper-V grew 215% year over year; and Citrix XenServer --which held fifth place in market share behind Microsoft's Virtual Server 2005 -- grew 290%. IDC research also indicated that for the full year of 2009, Citrix increased its overall market share most significantly from the previous year, from 1.8% to 6.3%.

Citrix's 'gateway to XenServer: XenApp and XenDesktop
Many users that have deployed XenServer over the past two years are existing XenApp or XenDesktop customers. These users, which had come to trust the Citrix technology over time, were more comfortable using XenServer to virtualize XenApp and XenDesktop servers. Pacer Hibler, a network engineer for New Hanover Regional Medical Center in Wilmington, N.C., wanted to add XenDesktop instances to an existing 1,700-client XenApp environment. "It made sense to stick with Citrix for desktop and endpoint virtualization -- I consider Citrix at the top of the market for desktops."

Some users' future virtualization growth may mean even more XenServer in their shops. ITHibler, who also manages about 200 VMware virtual machines on 15 physical hosts, said that in the future it might be cheaper for his organization to use 100% XenServer because it already has a Citrix XenApp Platinum license. "We're always looking to save costs and we already have the Citrix investment," he said. "If we keep growing the way we have been and VMware continues with the same costs, that could be the way we go."

Cost and consolidation for small shops
If users came to XenServer for the desktop, some have stayed because of pricing. Particularly for smaller shops, XenServer's free hypervisor offers the most-desired features.

If users came to XenServer for the desktop, some have stayed because of pricing.



Over the past year, Brad Williamson, a network administrator for a banking software company in southwest Florida, has converted from a mixed VMware/XenServer environment to an all-Citrix shop. Like other users, Williamson said his deployment began with desktops, but grew with new demands to expand virtualization to Web hosting without a budget.

XenServer and VMware both offer free versions of their hypervisors, but Williamson said the free version of XenServer includes live migration (known as XenMotion) and multi-server management. VMware's free ESXi hypervisor does not support managing multiple virtualized physical hosts simultaneously. As of vSphere 4.1, which extended features such as VMotion to lower vSphere license tiers, the lowest licensing level that includes VMotion is Essentials Plus, which costs $3,495 for up to three hosts and six processors.

The free version of XenServer doesn't include high availability, which Williamson said he "fakes" with replication at the storage and host level. "I haven't felt the need to go back to VMware at all."

The XenServer 5.6 Advanced edition includes HA, memory optimization, and performance alerting and reporting for $1000 per server; VMware HA also requires an Essentials Plus license.

Tiered-hypervisor environments growing more common

More often than not, expanding XenServer deployments sit alongside servers running VMware or Microsoft Hyper-V.

Phil DeMeyer, an information technologist at an early-childhood education program in the Midwest, chose to deploy vSphere, Hyper-V and XenServer according to what he saw as their natural affinity for different applications. "Existing integrations are more valuable to me than one throat to choke. I use the server virtualization solution that best fits a specific problem," DeMeyer said.

Hyper-V is used with complex Microsoft applications such as SharePoint Server, where DeMeyer says in his experience there's more direct integration between Hyper-V, the Windows operating system and Microsoft applications than with third parties. DeMeyer said the selling point for him to run XenApp on XenServer was the fact that he can buy XenApp as a pre-packaged XenServer virtual appliance. The remainder of DeMeyer's applications run on VMware, in part because it supports more storage, server and networking hardware options than its competitors..

For Ryan Koelewyn, an IT network analyst in California's public sector, VMware is deployed for production applications that need high availability. Less critical apps, like his shop's lab, go on XenServer along with XenApp. "HA isn't available in the free version of XenServer, but the majority of apps we run on XenServer don't require it."

The cons of XenServer
While users say they're satisfied with XenServer, it's unlikely to stage a takeover of their shops anytime soon. For one thing, management tends to lean in favor of the market leader. "I prefer XenServer because it's easy to get into -- it's more of an open system," said Koelewyn. "But we'll probably keep VMware -- upper-level management feels much more comfortable with it."

I prefer XenServer because it's ... more of an open system


Ryan Koelewyn, IT network analyst
California public sector

Users also say they want Citrix to continue developing XenServer's memory management. Today, the Citrix approach to memory management, called dynamic memory control, increases the number of virtual machines (VMs) on a given host by compressing the memory pages used by existing VMs. It's not quite the same as VMware's memory overcommitment, which essentially allows memory to be "thin provisioned" so guests behave as though they have more memory available than they do in reality. VMware also has transparent page sharing, which "single instances" memory pages, and with vSphere 4.1 added memory compression similar to XenServer's.

VMware-style memory overcommit is the one big feature XenServer is missing, according to Robert McNutt, a lead Citrix specialist at Northeast Georgia Health System Inc in Gainesville. "They've added a measure of memory management recently, but it's something VMware has been doing for years."

Like Hyper-V users, XenServer shops say more advanced integration with hardware and ISV partners is needed. For example, McNutt's primary storage array, an EMC Corp. Symmetrix DMX-4, is not on the hardware compatibility list for Citrix's StorageLink program, which allows management of storage arrays through the virtual server interface.

New Hanover Regional Medical Center's Hibler added that plans to expand his XenServer deployment will be contingent on support from his clinical application vendors, many of which have just begun to support virtualization and typically do so only on VMware.

Beth Pariseau is a senior news writer for Write to her at

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