XenSource's XenEnterprise

Virtualization will inevitably shrink the bite hardware takes out of our capital budgets. But VMware has somewhat dampened IT's enthusiasm by charging $3,000 per socket for its enterprise-class VMware ESX. Doesn't, say, $750 per perpetual dual-socket license sound a lot better?

At that price, XenSource's XenEnterprise 3.2 is an easy-to-install bargain that takes advantage of the open source Xen 3.04 hypervisor. For many organizations itching to get going with virtualization, XenEnterprise will serve nicely thanks to its solid performance and general ease of use. The current version has some drawbacks: For one, it doesn't yet support 64-bit Windows, but XenEnterprise 4.0 will and it's heading into beta now, with an expected mid-August production date.

Adding to market pressure, Microsoft, another latecomer to the virtual machine party, will include a sufficiently robust virtualization offering as part of its new server operating system. In what has to be good news for XenSource, the big guns in Redmond have preannounced formal support and integration for Xen-based VMs as part of the next server build, to optimize Windows Server 2008 to run on Xen and to let XenVMs run on Server 2008. XenSource is partnering with Microsoft to optimize Win/Xen and Xen/Win performance.

Ain't competition grand?MAKE THE MOVE
IT groups that plan frequent shuffling of VMs from host to host and need robust snapshot and rollback functionality will want to wait for the 4.0 release of XenEnterprise, which is based on the open source Xen 3.1 hypervisor. But for those who simply need to virtualize non-disk-intensive Linux environments or have a number of infrequently used 32-bit Windows 2003 application servers that occasionally demand high CPU performance, XenSource's XenEnterprise makes sense now.

If you have four, 10, even 15 single-purpose application servers humming away near end of life but have been holding back on virtualization because of license and support costs, you'll want to look at XenEnterprise. It's especially well-suited for shops that run a mix of Linux and Windows on Intel VT- or Advanced Micro Devices V-equipped servers and have in-house Linux talent. We found its hypervisor optimization and paravirtualization tools for Windows impressive.

And XenSource has solid credentials. It was founded by Ian Pratt and the University of Cambridge team that initially developed the open source Xen Project. A raft of venture funding, including the deep pockets of Kleiner Perkins Caufield & Byers, has let Pratt & Co. commercialize its success while continuing to support the open source initiative.

Speaking of vendor viability, as we reported recently, virtualization is shaping up to be a nightmare for server hardware vendors (see "IDC: Virtualization, Multicore Shake Up Server Market," March 21). Both Gartner and IDC forecast that VM software use could shove the compound annual growth rate for x86 servers into negative territory by 2010.

We agree that the trend to consolidate light-use and "single-box" servers will affect sales, but in the long run, we see Dell, Hewlett-Packard, IBM, and others offsetting potential lower volume with higher per-unit revenue as customers order larger, more robust servers purposed for virtualization in place of aging one-trick 1U ponies.BUT DOES IT WORK?
After running a battery of tests, we were impressed with the outstanding performance of the slim XenEnterprise hypervisor and XenSource's paravirtualization approach for hosting Windows Servers--at times running a truly impressive 99.9% of bare-metal CPU performance (see "Tester's Notebook: Ready, Get Set, Run").We tested XenEnterprise at our Real-World Partner Labs and found that XenSource has optimized the open source Xen 3.04 hypervisor into a tight, 50,000-line code package for running Microsoft operating systems as well as Linux distros. This design lets the virtualization engine hit bare metal for increased performance and contrasts sharply with the Microsoft and VMware model of running client operating systems on a traditional "fat" host OS.

As more security fears surface around virtualized environments, and virtualization management and security toolsets struggle to keep up with deployments of these environments, we believe that Xen's tight code base, coupled with open source peer review of the hypervisor, will yield glad security tidings.

The pricing news is pretty good, too. XenSource offers a gratis version of XenExpress that supports four XenVMs on one box. XenServer, for $99 annually, lets you host eight XenVMs on one server. The premier product, XenEnterprise 3.2, which is shipping now, is priced at $488 for an annual license or $750 for a perpetual license per dual-socket server.

Neither of these license models includes formal support beyond self-help tools and discussion forums, though XenSource--no doubt mindful of the pall a lack of service contracts casts over enterprise adoption--will provide support for $600 per server per year. You may not need it, however: Like most mainstream open source initiatives, the Xen community has a large peer-to-peer support structure in place. We relied extensively on the knowledge base and discussion forums as we worked through testing for this article, to see what those opting for open support can expect.

(click image for larger view)STORAGE QUESTION
The Achilles' heel of Xen, and to some degree all virtualized environments, is the shared disk pool hosting multiple virtual machines. The underlying concept of resource sharing across VMs on the same box plays well through CPU, cache, and memory. The hypervisor is able to allocate slices of CPU time as required by guest operating systems and their applications, and assuming adequate physical RAM is available, each VM can run at close to bare-metal performance during a normal workload distribution. Sure, you'll see a global performance hit when all VMs are running full tilt, but day-to-day operations with varied demands yield adequate performance for all.The virtualization benefit decreases when systems are short on physical memory and/or are running disk-intensive apps from a shared repository. VMware supports multiple repositories, providing far greater flexibility in host server design and off-box storage options. In contrast, all current XenSource offerings are limited to one storage repository per physical server. This can severely impact performance when two or more resource-intensive app servers start competing for the same disk pool.

In our loaded tests, we found that scores on our PassMark Software PerformanceTest hardware benchmark dropped significantly because of reduced disk performance scores, though the hypervisor does a solid job maintaining and allocating CPU and memory resources across the XenVMs (for full benchmarks, go to "XenSource Performance Numbers"). The PassMark disk scores reflect the contention in the disk channel for shared access to the drive array by our hosted sessions; the raw seek/ read/write performance of the serial-attached SCSI array hasn't changed, but the net result was poor performance for any given guest OS.

Our testing with a large iSCSI array shows that disk performance can be improved, but it will be difficult, if not impossible, to achieve near-native disk speeds in a shared repository with this version of XenEnterprise.

The moral of the story? Run disk input/output-dependent, performance-driven apps on native iron and wait for virtualization vendors to improve this area. The pending August release of XenEnterprise 4.0 is slated to allow 128 mounted repositories per server, which should directly address these concerns by letting customers run multiple low-disk-use servers in shared repositories while dedicating pools to disk-intensive VMs as required.

To see whether throwing expensive hardware at the problem would help, we attempted to mitigate disk performance issues by running the storage repository from a quick onboard six-drive RAID array and from a $70,000, EqualLogic 16-drive iSCSI array. Alas, neither gave us satisfactory performance with disk-intensive apps. The very robust iSCSI array wasn't able to overcome the limitations inherent in the shared repository model, though it did eke out marginal improvements over the internal array.Allowing multiple mounted repositories on different physical storage platforms will alleviate some performance issues and greatly improve hosts running multiple concurrent disk-intensive servers. We can extrapolate that virtualized disk performance would be similar to our single-guest test, roughly 40% of native disk performance for a similar dedicated storage setup, and look forward to future offerings from XenSource providing this functionality.

So, is this version of XenEnterprise right for you? We'll resist the Zen puns that come to mind and simply say, perhaps.

If disk-intensive operations rate high on your requirements list, any virtualization option should be addressed with a critical eye. If you need to move now and are considering VMware for its mature management features and live migration of VMs from host server to host server, Xen isn't for you.

But if you want to virtualize your non-disk-intensive Linux environment or have a number of infrequently used 32-bit Windows 2003 application servers that occasionally demand high CPU performance, XenSource offers an excellent optimized hypervisor and paravirtualization tools for Windows operating systems at a hard-to-beat price point. If it can deliver as promised with version 4.0, the company will move closer to VMware in a feature-for-feature comparison.

Joe Hernick teaches computer science at Suffield Academy; previously he was director of IT at the Loomis Chaffee School and technology services manager for a Fortune 100 company. He writes for InformationWeek as part of For IT, By IT, which provides firsthand analysis and insight from IT pros into technologies critical to enterprise IT. Write to him at [email protected].

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XenSource Performance Numbers