From Our Lab: XenSource's XenEnterprise Is A Virtualization Bargain

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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